The summary.
READ THE TERMS ON SUSPENSION AND FUND HOLDS BEFORE ANYTHING ELSE
Withheld settlement is an existential risk for a small business, and rate is a lesser consideration than settlement timing.
Plan a second provider — an outage at one is an outage of your revenue.
CREATE THE TRANSACTION SERVER-SIDE AND VERIFY IT WITH THE PROVIDER BEFORE FULFILLING
Never trust amounts, references or status from the browser, and never fulfil on the redirect alone.
MAKE NOTIFICATION HANDLERS IDEMPOTENT AND FAST
They are retried and arrive several times. Respond immediately and queue the slow work, or you generate duplicates.
Log every notification raw — it is the evidence when a payment is disputed internally.
EVERYTHING EXCEPT THE SUCCESS PATH IS COMMONLY UNTESTED
And the failure paths are where money is lost. Test declines, timeouts, duplicates, refunds and reversals — then make one small live transaction before launch.
RESOLVE EVERY PENDING TRANSACTION
Each one is a customer in an unknown state. Never retry an unknown outcome at another provider blindly.
