[sweetwater]

Disclaimer: I don't operate a business.
My thoughts are that a security camera that is aimed at the shop register and door may reduce insurance premiums, and that would be a friendlier way to accomplish the monitoring you need to stop or prove stealing from the register. An alternative - and the reason for the disclaimer - wouldn't numbered receipt books serve to document the money that should go in the register? Each customer is given a statement to pass to the cashier, and you have a record of how much money should be in there and a way to track misdirected funds.