When I need extra help with expenses, I run specials to cover them. I have had times when a guy will prepay for a future session, and I follow through. I realize that some ladies may not, but that is bad TCB. It doesn't matter if the guy prepays to help with an expense or at the time of the session, it is still paid for and the lady needs to follow through.
But I would prefer to get the money at the time of session, not have the guy pay for whatever my expense is.