Depends on a lot. What work are you having done? If they're making significant mods to the boot (punch/grind, replacing/removing material, etc. etc.) and basically going well above and beyond just a simple custom footbed, I'd hook em up—especially if it's a shop you frequent. People in shops and the ski industry in general get paid meager wages and often deal with some of the most entitled and self-absorbed people on the planet. Treat them well, and they will never forget it.
To me, a 6 pack of decent beer is a standard tip for pretty much any/all kinds of repair work, so at minimum, do that. I've tipped people as much as $20 before though. It's all relative. The fact that you're even posting this thread tells me that whatever you decide will be the right thing and they will take good care of you.