When I ask to substitute something, don't tell me you can't do it, just find a way to do it and charge me.

Last night I was at a Thai restaurant and I asked for drunken noodles with duck. It wasn't on the menu but I was willing to pay whatever upcharge they wanted. I work in a restaurant and they had the same point of sale system we use, I knew there was an "OPEN INFO" button under which you can enter whatever information you want but the waitress first told me it wasn't possible, then said she'd go ask for me. Thankfully, they eventually agreed to do it, but I think their thought process should be about saying yes rather than no.