Many galleries, reps, agents and such will pay 30, 60, 90 or even more days AFTER they have themselves been paid. This can run into months where one delivers a print to a gallery and the gallery is having and/or doing the framing, hanging and such. This is particularly true where the end client might be a large corporation with lots of accounts payable bureaucracy.
Where a special print order is made (as opposed to a print that the gallery has on hand), it is customary to ask for a deposit, 50% is normal. I'd think 50% would cover all expenses at the very least. If not, asking for at least the expenses as the deposit amount is reasonable in most cases.