A stamp card is the simplest loyalty scheme a shop can run. Each qualifying visit or purchase earns one stamp, and when the customer reaches a set number, called the target, they receive a reward that was stated before they started collecting. The count then goes back to zero and the next card begins. There are no balances to convert and no tiers to climb, so the customer can see how far off the reward is at a glance, and so can whoever is at the till. The card itself can be paper marked with an ink stamp, a card with holes punched in it, or a count kept against the customer's mobile number.
A stamp card works when three things are fixed and easy to say out loud: what earns a stamp, how many stamps fill the card, and what the reward is. Change any of them often and regulars stop trusting the count.
In practice
A cafe runs a nine-stamp card, one stamp per coffee, with a free coffee as the reward. A regular who buys a coffee every weekday for 46 working weeks buys 230 coffees, which fills 25 cards with a few stamps left over. Each free coffee costs the cafe about $1.50 in beans, milk and cup, so that regular's 25 rewards cost roughly $38 for the year, set against $1,150 of coffee at $5 a cup.
The same card would fail at a florist. A customer who buys a bunch every fortnight would need 18 weeks to collect nine stamps, long enough to forget the card exists. A florist sets five stamps, one per bunch, and makes the reward a bunch up to a set price.
In Counter Club
The card is two fields on the Loyalty card page: a target from 2 to 50 stamps, and the reward in your own words. Each press of Stamp on a member's row adds one stamp, and a full card produces a reward code and starts again. See the stamp card.