If you suspect any tax fraud, then go to the IRS and suggest an audit.

If you want a register receipt, then wait til the owner is there and have a polite chat and ask that the employee give you a register receipt.

Solve your problems from a position of power. the employee really is at the bottom of the food chain. So go through the manager/owner.

But - the handwritten receipt *is* good as evidence as suggested above

Best regards,
006mom