QR codes that actually get scanned

A QR code on a wall behind the register gets ignored. A QR code on a small stand at eye level, where the customer is already waiting, gets scanned.

Put codes where hands and eyes already are: the counter, the payment terminal, the invoice, the receipt, the packaging, the service vehicle door.

Always give the code a reason. "Leave us a review" outperforms a naked square every time, because a naked square is a mystery and people do not scan mysteries.

Finally, test it on a cheap phone in bad light. If it hesitates, make the code bigger.