My DH has a tree service and has sometimes gotten tips.
Sometimes people give them drinks and food too. It is a really nice gesture and so very appreciated on the hot days to have cold drinks. But they always bring whatever they need as well...so no extras are ever necessary. They bid for what they need to get and it is expensive to have trees down so extras add up even more.
As a side note, anyone needing trees completely removed always get more than one estimate to compare and make sure they are insured, and if they have employees that they have workmens comp on them. If you are getting pruning or shaping done, seek a ISA Certified Arborist to do that work, as it truly is a specialty that needs to be done right and a lot of guys that do removals aren't skilled in pruning as much and can do more damage in the long run. And topping a tree----please don't ever do that. It is so bad for trees.