Amounts and orders

Hold money as integers with a currency, freeze what the customer agreed to, move orders through fixed states and never charge the same order twice.

By Erik Bergstedt, Agent Wranglers. Updated 20 October 2026.

Required

Hold money as integer minor units everywhere
Required
Hold every amount as an integer count of the currency's minor unit, with its ISO 4217 code, in code, database, provider calls and stored events.
Freeze what the customer agreed to on the order
Required
When the order is created, copy the unit price, quantity, discount, tax rate, tax amount and currency onto each line. Never recompute them from the catalogue.
Move orders through a fixed set of states
Required
Give orders a fixed list of states and allowed transitions in one place, and refuse any other change with a conditional database update.
Create each charge with an idempotency key
Required
Send an idempotency key, built from your own stable IDs, on every provider request that creates a payment, a refund or a payout.
Keep one open payment per order
Required
Allow at most one open payment attempt per order, and reuse it when the customer comes back to checkout.