Your six-stamp coffee card, on their phone

Same deal as the paper card: six coffees and the next one is on the house. The card lives in the customer's Apple Wallet or Google Wallet. Your staff scan it instead of stamping it.

1. They add it once. A QR sign on the counter, one tap on Add to Wallet. No app to download.
2. Staff scan it. Each coffee is one scan with a phone or tablet behind the counter. The card on their phone updates straight away.
3. The sixth coffee unlocks the free one. Their phone shows it ready, staff scan to give it and the card starts again.

Try it: press Scan card on the counter screen and watch the card fill up.