This may help you. So we have a Car Wash and we offer an Unlimited wash service. I have an item for the level of service and the price that we charge when they purchase the service. We then had custom business cards made up as their membership card. On the back, we add a label using a brother label printer, (works with iPad) and put their name and expiration date on it. They present this every time.
When they come in for a wash they present the card we look up there name and ring a "Usage" item. This is an item with a zero price. This way we can track how many we do as part of this program. In your case you may want to add a Coffee Club Discount, so you can track coffee type.
Now the other thing we have done is put these customers in a special group so we can manage them better. We also added an expiration date as a customer profile. In our case, we offer 6 months and 12 months memberships.
