[Musiclab]

Speaking for myself, recently I mixed a project for a guy from England.
He sent me the tracks of his song and after a discussion about what he was looking for I gave him an estimate and he sent me half up front. Then I mixed the song, sent him an mp3 and when I received the balance I sent him the Hi res mixes. I don't know that I would be Ok with this every time, but it worked this time.
You're soliciting the mixer, he has to take time to do your mix, during which he's not getting paid, if you were to skip out on the bill. If you are pals it's one thing but if you don't know each other, and the mixer is reputable it's shouldn't be a problem. I would ask for a completion date.