Per https://solink.com/resources/top-types-employee-theft/ it’s basically “Skimming”:

This popular POS employee theft scam occurs when an employee charges a customer full price but takes a little cash out of the total for themselves. This may result in telltale cash register imbalances, or, it can be more complex, like using coupon codes on a customer’s purchase while still charging them full price and then pocketing the difference.

Your implementation sounds pretty close to the coupon version.

Are your bosses ever going to notice? Unlikely. And if they did they’d probably fire you at worst, rather than press charges.

Now, that said, if your manager or employee handbook or any other source of authority at your job says it’s okay, then it is (excepting, of course, if they explicitly tell you it isn’t) - but that’s because it’s a benefit your employer has given you rather than one you’ve taken.