Subscriptions, Invoicing & Dunning

Subscriptions add plans/prices, billing cycles, trials, proration on upgrades/downgrades, invoices, taxes, and dunning (retrying failed renewals with smart schedules, card updater, emails). Use a billing system (Stripe Billing, Chargebee, Recurly, or the PSP's) rather than building one, and keep entitlements (what the customer can use) in your app, driven by billing events (invoice.paid, customer.subscription.updated/deleted). Decide grace-period policy: how long access continues while payment is past due.

Build vs buy billing

Python (entitlements from events)

async def on_billing_event(event):
    sub = event.data.object
    account = await accounts.by_customer(sub.customer)
    if event.type in ("customer.subscription.created", "customer.subscription.updated"):
        await entitlements.set(account.id, plan=sub.items.data[0].price.lookup_key,
                               status=sub.status,                      # active | trialing | past_due | canceled…
                               current_period_end=sub.current_period_end)
    elif event.type == "customer.subscription.deleted":
        await entitlements.downgrade_to_free(account.id)
# Access check: active/trialing → full; past_due → grace period banner; canceled → free tier

Common pitfalls

• Checking the PSP on every request for access (latency, rate limits).
• Immediate lockout on first failed renewal.
• Ignoring proration and tax on plan changes.

Key points

• Use a billing platform
• Entitlements in your app, driven by events
• Explicit dunning and grace policy

Further reading: Stripe Billing: How subscriptions work