Business ethics.
For 'Hands on' ... the problem with your system is the 'goodwill' factor.
Over the years and in my business, I decided I was never prepared to treat my every day customers as '!!!!s' and demand money up front for my service, just because one in a hundred of them was a defrauding !!!!. In thirty years I've never solved that problem and like many others, the rotten 'one in a hundred' has taken me for a few hundred and a few thousand and in one case half way to six figures.
Cest la vie .... Dennis Kenyon.