To mail it himself would take some long-term planning. He'd have to book off a time when he knows he'll have the bill and the address with him, not put this off

To mail it himself would take some long-term planning. He'd have to book off a time when he knows he'll have the bill and the address with him, not put this off until tomorrow, locate and go to a post office/convenience store to buy stamps and an envelope, get the money in his account, and then keep it there for five or six business days until he's sure that it's been cashed. If you're living paycheque-to-paycheque, this is more difficult than you would think - especially the part about having the fifty bucks sitting in his account for five days without touching it.