Upload & Sell: Off
I actually just spoke with a successful commercial photographer about this. He covers the usage terms by stating them as a line item on his invoice and requiring full payment before the terms go into effect (also stated on the invoice). So, the client agrees that they cannot use the images until they pay him, giving him something to take into court if they try to stiff him.
Maybe you wouldn't be comfortable without a full-blown contract, but that's the way this guy does it and he's pretty much working at the top of his field.