The warranty call ends with a diff, not an argument

Three weeks after handoff, the call: checkout is broken, fix it, it's under warranty.

On handoff day you took a snapshot and aliased it delivery. Now you copy a new container from that snapshot, and the machine you handed over boots next to the machine they've been running. Checkout works on the delivery copy. Then the file journal, queried between handoff day and today: Tuesday, 4:12pm, the payment config was edited: by their contractor.

You send both facts, not an accusation: works at delivery, changed on Tuesday. They pay for the fix, and the relationship survives, because nobody had to win a memory contest.