Subscription Billing: What Actually Breaks When You Build It Yourself

A one-time payment is a single event: charge the card, confirm the order, done. A subscription is a state machine that runs for years, and most of the bugs in home-grown billing systems live in the transitions nobody designed for on day one — a customer upgrading mid-cycle, a card expiring the week before renewal, a refund request three months after a plan change.

Where in-house billing systems actually break

• Proration on plan changes — upgrading or downgrading mid-cycle means calculating a partial credit and a partial charge, and getting this wrong either overcharges customers or silently leaks revenue
• Failed payment recovery — a card decline isn't a single event to handle, it's a retry schedule (dunning) with its own timing, customer notifications, and a decision about when to actually suspend access
• Timezone and billing-date drift — a subscription that renews on the 31st behaves differently in February, and naive date-math implementations accumulate small errors that show up as support tickets months later
• Tax and currency changes mid-subscription — a customer relocating or a tax rate changing between billing cycles needs to be applied to the next invoice correctly, not retroactively or not at all
• Idempotency — a webhook or retry firing twice must not charge the customer twice, which requires deliberate idempotency keys, not "it probably won't happen twice"

Build vs buy, honestly

Stripe Billing, Chargebee, and similar platforms exist because this problem has already been solved correctly, at a level of edge-case coverage that's expensive to replicate. The build-it-yourself case only holds up when your pricing model is genuinely unusual — usage-based billing with a highly custom metering logic, or billing tied tightly to a proprietary internal ledger — and even then, most teams are better off using a billing platform's API for the payment and invoicing mechanics while keeping the custom logic in their own application layer.

The cost comparison people get wrong is licensing fees versus "free" in-house code. The in-house version isn't free — it's an ongoing engineering cost for every edge case above, indefinitely, paid in either engineering time or in the customer trust lost when billing is visibly wrong.

What to get right regardless of which you choose

• Treat every plan change as an auditable event, not a silent field update — you will need the history when a customer disputes a charge
• Build the dunning and cancellation flow before launch, not after the first wave of failed payments arrives
• Reconcile your billing system against your bank deposits on a schedule — mismatches compound quietly if nobody checks

The short version

Recurring billing is a state machine with more edge cases than it looks like from the pricing page. Use a platform built for it unless you have a specific, well-understood reason not to, and design the failure paths — declined cards, disputed charges, mid-cycle changes — before launch, not after they show up as support tickets.

Sources