Five of the checks from the billing kit, running against a simulated billing engine with a clock you control. Nothing leaves your browser.
Do this. Apply coupon SAVE20 to a subscription, then apply the identical coupon again before the invoice is issued.
Nobody tests applying a coupon they have already applied, because in the interface the field is cleared. The API does not clear anything.
Do this. Make the card decline, advance past every retry in the dunning schedule, then read the entitlements.
The billing form of a declined payment creating an order: the money never arrived and the service is being delivered anyway, every month, silently.
Do this. Cancel, then advance the clock past three renewal boundaries — ninety days, not thirty.
This is the billing defect most likely to reach social media. A customer who cancelled and was charged anyway does not open a support ticket first.
Do this. Apply a three-month coupon and advance past four renewals. Read all four invoices, not the first one.
Revenue leaking on a schedule nobody is watching. The first three invoices are correct, which is exactly why this survives release.
Do this. Upgrade mid-cycle while a 20% discount is active, and recompute the invoice by hand.
Proration arithmetic is the one place where doing the sum by hand, once per release, finds more than any other habit on the list.
Card charged during the free trial · unlimited trials per customer · renewal charges the old price · a renewal charged twice · month-end dates skip short months · upgrade charges full price with no credit · downgrade refunds immediately · dunning retries forever · cancellation revokes access immediately · refunds exceed the amount paid · VAT calculated before the discount · entitlements not reduced after a downgrade · recognised revenue does not match the charges taken.
All eighteen are armable individually in the sandbox that ships with the kit.
Get the free checklistThis page runs entirely in your browser. There is no server, no account, no data stored and nothing transmitted. The billing engine is simulated in JavaScript so the defects are safe to demonstrate.