Here's the thing: no matter how much you paid for your ticket, you never bother to even try bullying the conductor or the gate agent to get your train or plane to leave a few minutes later.
It leaves when it leaves, that's the deal.
Part of the challenge of selling custom work is that it sometimes seems that everything is up for grabs. You should stay up all night for a week. You should rearrange the orchids in order of smell, because even though it's not in the spec, hey, that would be good service, and we are paying a lot…
Promising perfect is actually not nearly as useful as promising what the rules are.
Boundaries eliminate the temptation to bully. State them early and often and don't alter them and believe it or not, the client will be happier as well. They didn't sign up to ruin your life. They signed up to get the most they could from you and your team, and the limits are the limits.