Why can't you? There's no rule or law says your business can't set it's own policies. Granted many hiring parties would not be happy with it but here are some jobs I would NOT extend credit for - loan application signings, Structured Settlements, Debt Resolutions - I will not make my fee contingent on any deal being completed.
I don't do Structured Settlements OR Debt Resolutions, but my fee for loan apps is payable in advance. Most companies pay when the loan funds - well, guess what, there's no guarantee it ever will - and you're left out in the cold.
Anyway, that said, point is there's nothing stopping you from establishing your companies policies - and one of them would be fee up front.