Unless it is really secure, don't allow your customers to leave cash around.
I had one customer who left £40 cash outside, and it wasn't exactly visible to passers by either, but you guessed it, it went missing
. Thing is, customer hadn't told me that she was leaving it out for me, so when she had a card through her door telling her she owed me £40. She wasn't best pleased as you can imagine. Made it very difficult to speak to her, and for some reason I almost felt guilty, but at the end of the day I didn't tell her to leave it out, so not my responsibility. Harsh, but true.