Billing Integration Skill

Patterns for integrating payment processing and subscription management into SaaS applications.

When to Use This Skill

Use this skill when:

• Billing Integration tasks - Working on payment processing and subscription management integration patterns. covers stripe, payment lifecycle, webhooks, dunning, and billing system architecture
• Planning or design - Need guidance on Billing Integration approaches
• Best practices - Want to follow established patterns and standards

Overview

Billing integration connects your application to payment processors (Stripe, etc.) for subscription management, payment collection, and revenue operations.

Billing Architecture

+------------------------------------------------------------------+
|                    SaaS Billing Architecture                      |
+------------------------------------------------------------------+
|                                                                   |
|  +----------------+    +-----------------+    +----------------+  |
|  | Your App       |--->| Billing Service |--->| Stripe/Payment |  |
|  | (Subscriptions)|    | (Orchestration) |    | Provider       |  |
|  +----------------+    +-----------------+    +----------------+  |
|         |                      |                      |           |
|         v                      v                      v           |
|  +----------------+    +-----------------+    +----------------+  |
|  | Entitlements   |    | Usage Metering  |    | Webhooks       |  |
|  | (Feature Flags)|    | (Consumption)   |    | (Events)       |  |
|  +----------------+    +-----------------+    +----------------+  |
|                                                                   |
+------------------------------------------------------------------+

Integration Patterns

Stripe as Source of Truth

Pattern: Stripe owns subscription state
Your App: Syncs state via webhooks
Benefit: Stripe handles complexity (proration, dunning, taxes)
Caveat: Need reliable webhook processing

Hybrid Approach

Pattern: Your DB + Stripe in sync
Your App: Owns business logic, uses Stripe for payments
Benefit: More control, faster reads
Caveat: Must keep in sync (eventual consistency)

Core Entities

Billing Domain Model

// Your internal representation (synced from Stripe)
public sealed record Subscription
{
    public required Guid TenantId { get; init; }
    public required string StripeSubscriptionId { get; init; }
    public required string StripePriceId { get; init; }
    public required SubscriptionStatus Status { get; init; }
    public required DateTimeOffset CurrentPeriodStart { get; init; }
    public required DateTimeOffset CurrentPeriodEnd { get; init; }
    public DateTimeOffset? CancelAt { get; init; }
    public DateTimeOffset? TrialEnd { get; init; }
    public required int Quantity { get; init; }  // Seats
}

public enum SubscriptionStatus
{
    Trialing,
    Active,
    PastDue,
    Canceled,
    Unpaid,
    Incomplete,
    IncompleteExpired,
    Paused
}

public sealed record BillingCustomer
{
    public required Guid TenantId { get; init; }
    public required string StripeCustomerId { get; init; }
    public required string Email { get; init; }
    public string? DefaultPaymentMethodId { get; init; }
    public required string Currency { get; init; }
    public TaxInfo? TaxInfo { get; init; }
}

Stripe Integration

Client Setup (.NET)

// Program.cs - Register Stripe client
builder.Services.AddSingleton<IStripeClient>(sp =>
{
    var apiKey = builder.Configuration["Stripe:SecretKey"];
    return new StripeClient(apiKey);
});

// Register services
builder.Services.AddScoped<CustomerService>();
builder.Services.AddScoped<SubscriptionService>();
builder.Services.AddScoped<InvoiceService>();
builder.Services.AddScoped<PaymentMethodService>();
builder.Services.AddScoped<UsageRecordService>();

Creating Customers

public sealed class BillingService(
    CustomerService customerService,
    SubscriptionService subscriptionService,
    IBillingRepository repository)
{
    public async Task<BillingCustomer> CreateCustomerAsync(
        Guid tenantId,
        string email,
        string? name = null,
        CancellationToken ct = default)
    {
        var options = new CustomerCreateOptions
        {
            Email = email,
            Name = name,
            Metadata = new Dictionary<string, string>
            {
                ["tenant_id"] = tenantId.ToString()
            }
        };

        var stripeCustomer = await customerService.CreateAsync(options, cancellationToken: ct);

        var customer = new BillingCustomer
        {
            TenantId = tenantId,
            StripeCustomerId = stripeCustomer.Id,
            Email = email,
            Currency = "usd"
        };

        await repository.SaveCustomerAsync(customer, ct);
        return customer;
    }
}

Creating Subscriptions

public async Task<Subscription> CreateSubscriptionAsync(
    Guid tenantId,
    string priceId,
    int quantity = 1,
    bool startTrial = false,
    CancellationToken ct = default)
{
    var customer = await repository.GetCustomerAsync(tenantId, ct)
        ?? throw new InvalidOperationException("Customer not found");

    var options = new SubscriptionCreateOptions
    {
        Customer = customer.StripeCustomerId,
        Items =
        [
            new SubscriptionItemOptions
            {
                Price = priceId,
                Quantity = quantity
            }
        ],
        PaymentBehavior = "default_incomplete",
        PaymentSettings = new SubscriptionPaymentSettingsOptions
        {
            SaveDefaultPaymentMethod = "on_subscription"
        },
        Metadata = new Dictionary<string, string>
        {
            ["tenant_id"] = tenantId.ToString()
        }
    };

    if (startTrial)
    {
        options.TrialPeriodDays = 14;
    }

    var stripeSub = await subscriptionService.CreateAsync(options, cancellationToken: ct);

    return MapToSubscription(tenantId, stripeSub);
}

Webhook Processing

Webhook Handler

[ApiController]
[Route("webhooks/stripe")]
public class StripeWebhookController(
    IBillingWebhookHandler handler,
    IConfiguration config,
    ILogger<StripeWebhookController> logger) : ControllerBase
{
    [HttpPost]
    public async Task<IActionResult> HandleWebhook()
    {
        var json = await new StreamReader(HttpContext.Request.Body).ReadToEndAsync();
        var signature = Request.Headers["Stripe-Signature"].ToString();
        var webhookSecret = config["Stripe:WebhookSecret"];

        try
        {
            var stripeEvent = EventUtility.ConstructEvent(
                json,
                signature,
                webhookSecret);

            logger.LogInformation(
                "Processing Stripe webhook: {EventType} {EventId}",
                stripeEvent.Type,
                stripeEvent.Id);

            await handler.HandleEventAsync(stripeEvent);

            return Ok();
        }
        catch (StripeException ex)
        {
            logger.LogError(ex, "Stripe webhook signature verification failed");
            return BadRequest();
        }
    }
}

Event Handler

public sealed class BillingWebhookHandler(
    IBillingRepository repository,
    IEntitlementService entitlements,
    ILogger<BillingWebhookHandler> logger) : IBillingWebhookHandler
{
    public async Task HandleEventAsync(Event stripeEvent)
    {
        switch (stripeEvent.Type)
        {
            case Events.CustomerSubscriptionCreated:
            case Events.CustomerSubscriptionUpdated:
                await HandleSubscriptionChangeAsync(
                    (Stripe.Subscription)stripeEvent.Data.Object);
                break;

            case Events.CustomerSubscriptionDeleted:
                await HandleSubscriptionDeletedAsync(
                    (Stripe.Subscription)stripeEvent.Data.Object);
                break;

            case Events.InvoicePaid:
                await HandleInvoicePaidAsync(
                    (Invoice)stripeEvent.Data.Object);
                break;

            case Events.InvoicePaymentFailed:
                await HandlePaymentFailedAsync(
                    (Invoice)stripeEvent.Data.Object);
                break;

            case Events.CustomerSubscriptionTrialWillEnd:
                await HandleTrialEndingAsync(
                    (Stripe.Subscription)stripeEvent.Data.Object);
                break;

            default:
                logger.LogDebug("Unhandled event type: {Type}", stripeEvent.Type);
                break;
        }
    }

    private async Task HandleSubscriptionChangeAsync(Stripe.Subscription stripeSub)
    {
        var tenantId = Guid.Parse(stripeSub.Metadata["tenant_id"]);

        var subscription = new Subscription
        {
            TenantId = tenantId,
            StripeSubscriptionId = stripeSub.Id,
            StripePriceId = stripeSub.Items.Data[0].Price.Id,
            Status = ParseStatus(stripeSub.Status),
            CurrentPeriodStart = stripeSub.CurrentPeriodStart,
            CurrentPeriodEnd = stripeSub.CurrentPeriodEnd,
            CancelAt = stripeSub.CancelAt,
            TrialEnd = stripeSub.TrialEnd,
            Quantity = (int)stripeSub.Items.Data[0].Quantity
        };

        await repository.SaveSubscriptionAsync(subscription);

        // Update entitlements based on new subscription state
        await entitlements.SyncEntitlementsAsync(tenantId);
    }
}

Critical Webhooks

Event	Action Required
customer.subscription.created	Provision tenant, set entitlements
customer.subscription.updated	Update entitlements (plan change)
customer.subscription.deleted	Revoke access, cleanup
invoice.paid	Confirm payment, extend access
invoice.payment_failed	Notify, start dunning
customer.subscription.trial_will_end	Notify user (3 days before)

Payment Lifecycle

Subscription States

Lifecycle Flow:
                                   +-------------------+
                                   |                   |
                                   v                   |
+----------+    +----------+    +--------+    +-------------+
| Trialing |--->| Active   |--->| PastDue|--->| Canceled    |
+----------+    +----------+    +--------+    +-------------+
     |              |               |               ^
     |              v               v               |
     |         +---------+    +---------+           |
     +-------->| Canceled|    | Unpaid  |-----------+
               +---------+    +---------+

Key Transitions:
- Trialing -> Active: First successful payment
- Active -> PastDue: Payment failed (retry in progress)
- PastDue -> Active: Payment succeeded on retry
- PastDue -> Canceled: All retries exhausted
- Any -> Canceled: User or admin cancellation

Dunning (Failed Payment Recovery)

public sealed class DunningService(
    IEmailService email,
    IBillingRepository repository)
{
    public async Task HandlePaymentFailedAsync(
        Guid tenantId,
        string invoiceId,
        int attemptCount,
        DateTimeOffset nextRetry)
    {
        var customer = await repository.GetCustomerAsync(tenantId);

        // Progressive messaging based on attempt
        var message = attemptCount switch
        {
            1 => "We couldn't process your payment. We'll retry automatically.",
            2 => "Second payment attempt failed. Please update your payment method.",
            3 => "Final payment attempt upcoming. Update your payment to avoid service interruption.",
            _ => "Your subscription is at risk. Immediate action required."
        };

        await email.SendAsync(new PaymentFailedEmail
        {
            To = customer.Email,
            Subject = $"Payment Issue - Attempt {attemptCount}",
            Message = message,
            UpdatePaymentUrl = GenerateUpdatePaymentUrl(tenantId),
            NextRetryDate = nextRetry
        });

        // Log for support visibility
        await repository.LogDunningEventAsync(tenantId, invoiceId, attemptCount);
    }
}

Usage-Based Billing

Reporting Usage to Stripe

public sealed class UsageReporter(
    UsageRecordService usageRecordService,
    IUsageMeteringService metering,
    IBillingRepository repository)
{
    public async Task ReportDailyUsageAsync(DateOnly date, CancellationToken ct)
    {
        var subscriptions = await repository.GetMeteredSubscriptionsAsync(ct);

        foreach (var sub in subscriptions)
        {
            var usage = await metering.GetDailyUsageAsync(
                sub.TenantId,
                date,
                ct);

            foreach (var metric in usage)
            {
                var subscriptionItemId = sub.GetSubscriptionItemId(metric.MetricName);

                await usageRecordService.CreateAsync(new UsageRecordCreateOptions
                {
                    SubscriptionItem = subscriptionItemId,
                    Quantity = (long)metric.TotalQuantity,
                    Timestamp = date.ToDateTime(TimeOnly.MinValue),
                    Action = "set"  // "set" replaces, "increment" adds
                }, cancellationToken: ct);
            }
        }
    }
}

Checkout Flow

Stripe Checkout Session

public async Task<string> CreateCheckoutSessionAsync(
    Guid tenantId,
    string priceId,
    string successUrl,
    string cancelUrl,
    CancellationToken ct = default)
{
    var customer = await repository.GetCustomerAsync(tenantId, ct);

    var sessionService = new SessionService();
    var session = await sessionService.CreateAsync(new SessionCreateOptions
    {
        Customer = customer?.StripeCustomerId,
        CustomerEmail = customer?.Email,
        Mode = "subscription",
        LineItems =
        [
            new SessionLineItemOptions
            {
                Price = priceId,
                Quantity = 1
            }
        ],
        SuccessUrl = successUrl + "?session_id={CHECKOUT_SESSION_ID}",
        CancelUrl = cancelUrl,
        SubscriptionData = new SessionSubscriptionDataOptions
        {
            Metadata = new Dictionary<string, string>
            {
                ["tenant_id"] = tenantId.ToString()
            }
        },
        AllowPromotionCodes = true
    }, cancellationToken: ct);

    return session.Url;
}

Customer Portal

public async Task<string> CreatePortalSessionAsync(
    Guid tenantId,
    string returnUrl,
    CancellationToken ct = default)
{
    var customer = await repository.GetCustomerAsync(tenantId, ct)
        ?? throw new InvalidOperationException("Customer not found");

    var portalService = new Stripe.BillingPortal.SessionService();
    var session = await portalService.CreateAsync(new SessionCreateOptions
    {
        Customer = customer.StripeCustomerId,
        ReturnUrl = returnUrl
    }, cancellationToken: ct);

    return session.Url;
}

Best Practices

Idempotency

// Store processed webhook event IDs
public async Task<bool> TryProcessWebhookAsync(string eventId)
{
    // Check if already processed (idempotency)
    if (await repository.WebhookAlreadyProcessedAsync(eventId))
    {
        logger.LogInformation("Webhook {EventId} already processed", eventId);
        return false;
    }

    // Mark as processing (with TTL for cleanup)
    await repository.MarkWebhookProcessingAsync(eventId);
    return true;
}

Webhook Reliability

Recommendations:
1. Return 200 quickly (process async if needed)
2. Store raw event for replay/debugging
3. Implement idempotency (track event IDs)
4. Handle out-of-order events gracefully
5. Set up webhook endpoint monitoring
6. Use Stripe CLI for local development

Anti-Patterns

Anti-Pattern	Problem	Solution
Sync API calls only	Miss events, state drift	Use webhooks as source of truth
No idempotency	Duplicate processing	Track event IDs
Blocking webhook handlers	Timeouts, retries	Process async, return 200 fast
Hardcoded prices	Painful to change	Use Stripe price IDs, sync from Stripe
No retry handling	Lost events	Implement retry with backoff

References

Reference documentation for detailed implementation patterns (load on demand):

• usage-metering skill - Consumption tracking for usage-based billing
• subscription-models skill - Pricing tier design and feature bundling
• entitlements-management skill - Feature gating based on subscription

For Stripe-specific patterns, use MCP research:

perplexity: "Stripe .NET SDK billing integration patterns"
context7: "Stripe.net" (for SDK documentation)

Related Skills

• subscription-models - Pricing tier design
• usage-metering - Consumption tracking for usage-based billing
• entitlements-management - Feature gating based on subscription

MCP Research

For current billing integration patterns:

perplexity: "Stripe .NET SDK 2024" "SaaS billing integration patterns"
context7: "Stripe.net" (for SDK documentation)
microsoft-learn: "Azure SaaS billing" "subscription management patterns"

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Last Updated: 2025-12-29