Here's something else to consider -- accepting cash compounds problems when it comes to companies that accept cash for services rendered AND uses both invoices and sales slips.
Let's say a nonprofit is holding a fundraiser for a charity. Everyone who attends the event pays at the door. The money raised is recorded through custom sales slips. It's hard to get away with not recording checks and credit card transactions, but it's easy for the person handling the money to pocket a bunch of cash and simply never generate a sales slip for it.
Stealing from a charity is a horrible thing, but it happens when unscrupulous people can get way with it.