Digital stamp cards, start to finish.

It is the punch card mechanic with a different storage medium. A customer earns one stamp per qualifying visit or purchase, the card fills up, and the completed card is exchanged for a reward. Nothing about the psychology changes — the customer can see how close they are, and being close is what brings them back.

What changes is where the state lives. On paper, the record is the card itself, which means the customer is responsible for not losing it and the business has no record at all. Digitally, the record sits against the customer's account, so a lost phone is a login problem rather than a lost relationship.

The mechanic has to survive a queue. The version that works in practice is: the customer opens their card, scans the QR code at the counter, and a team member approves the stamp. That keeps the approval with staff — so a customer cannot stamp themselves from the car park — without adding a device or a step for the person paying.

Anything slower than that gets skipped. If issuing a stamp means typing a phone number, looking up an account, or waiting for an app to load, it will happen at 10am and not at the lunch peak, which is exactly when it matters most.

The number should be reachable inside a normal customer's ordinary habits. A café customer who comes three times a week fills a ten-stamp card in under a month, which feels like momentum. A restaurant a guest visits monthly would take most of a year to fill the same card, and momentum turns into indifference somewhere around stamp four.

A rough way to set it: take how often a typical regular already visits, and pick a number they can complete in four to six weeks.

A stamp card has more settings than it looks like, and most of the disappointing launches come from leaving one of them vague rather than from picking the wrong value.

The number that matters is not how many cards were handed out. It is how many were completed, and how the visit frequency of people holding a card compares to everyone else.

Give it a full cycle before you judge it. If a card takes six weeks to fill, six weeks is the earliest point at which the completion rate means anything at all.