Whatever envelope I find lying around. Normally I’ll stick it in the case with the chips - keeping the cash and unsold chips all together makes sense to me.
Registers and cash boxes are great if that’s how you want to do it. The only thing that turns me off is when the host jams the money in his own pocket. I don’t care for my money being in another player’s pocket. I realize that any decent host is not only going to pay out every penny, but also cover any shortage out of his own pocket, so maybe that’s where it actually belongs. But it just feels more comfortable to me if there’s more separation between the house and the guy playing at the table.
(And I realize that’s very silly, and it certainly doesn’t bother me THAT much, but if I get a twinge of that feeling, I’m sure there are other people who do too, so I feel like it’s worth mentioning.)