Which system has the final word?
For every important field, such as a price or a customer's address, decide which system owns it and which only receives a copy. Two systems that both think they are right will cause endless clean-up work.
What happens when it fails?
Services go down and passwords expire. A failed transfer should be visible to someone and safe to retry, without creating a duplicate order or payment.
Write the agreement down
List the fields, formats and identifiers each side expects, test with realistic examples and agree how either side will announce future changes.
Make problems easy to spot
Keep a clear log of what was sent and what failed, without storing sensitive details, and give one person the job of checking it.
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