I have a little one who uses formula. The parents send two empty bottles and a contained of dry formula. I just rinse the bottles out for them and they wash them at home.

As for heating up a bottle, I don't and I think it's much easier if you don't. The majority of kids will drink a bottle that isn't warm! I just keep my water on the counter so it's at room temperature--this way they aren't getting a cold bottle, but it doesn't need to be heated either.