Check the final bill against the record, not your memory

The final account - the build's last bill, where its whole money history gets settled - lands when you are most tired and least inclined to argue, and it is written by the builder from the builder's records. Before agreeing anything, put it against your own: the contract price, every change that was priced in writing, every payment already made, the snags still open. Concede what the record supports as readily as you refuse what it does not - a checked account cuts both ways, and that is exactly what makes it persuasive. What you never do is settle it from memory in a doorway conversation, however much you want the keys.