Subscription test cases, for the invoice that nobody can explain

Proration is arithmetic that has to be exact and is usually approximate. The unused portion of the old plan credited against the remainder of the new one depends on the daily rate, and the daily rate depends on the actual length of the cycle. An implementation that assumes a thirty day month is wrong in February and wrong in every thirty one day month, by an amount small enough that nobody disputes it individually and large enough to matter across a customer base. The renewal date has the same problem from the other direction: a subscription started on the thirty first has to land somewhere in a thirty day month, and it must not drift forward every cycle afterwards.

Webhooks are the integration surface where subscription systems most often break, because the delivery guarantees are weaker than they appear. Providers retry, which means duplicate delivery is normal rather than exceptional, so every handler needs to be idempotent on the event identifier. Delivery order is not guaranteed either, so an updated event can arrive before the created event, and applying whichever arrived last produces a state that matches neither. And the endpoint has to verify the signature, because an unauthenticated webhook handler lets anyone grant themselves a paid subscription with a single request.

The renewal job is the highest cost defect in the set and the easiest to miss, because it only fires on a schedule. Without a guard that ties a charge to a billing period, the first time that job is retried after a partial failure it charges every active customer a second time. It is testable by simply running it twice, which almost no test suite does.

Finally, failure states need to be designed rather than inherited. A declined renewal should move the subscription to past due and follow a documented retry schedule, and the access policy during grace has to be an explicit decision rather than a side effect of how the entitlement check happens to be written. Recovery matters just as much: updating a payment method on a past due subscription must restore access immediately and must not take a second charge. Every one of these transitions is a place where a paying customer either loses access they have paid for or keeps access they have not.