Here is some information I copied from the Princess board. It is about a coffie card/stamp You can use it at the internantional cafe. Hope it helps.
I have been on two Princess cruises this year, The purchase of coffee cards is the only way to go.
You can use the same card for every cruise, as long as you don't fill up all 15 punches on your card.
You get unlimited fresh brewed coffee and some of the best teas I have ever tasted.
The international Cafe is the location to get TO GO cups.
June 20th, I will be on a 14-day Alaska Cruise on the Diamond Princess, Vancouver r/t............... can't wait to use my existing coffee card.
----------------About the cost--------------
It costs $27.60 ($24 for the card + 15% tip). When you first embark, there will be various tables set up (usually near the buffet elevators) that are selling soda cards, ultimate kids package, and the coffee cards. Alternatively, you can just go to the International Cafe and buy the card there. That is where they have the fresh brewed coffee and all the specialty coffees. You can just go to either one and ask to purchase the coffee card.
A friend said they would go to the International Cafe for a coffee drink and the cafe waiter would bring them cookies or pastry at no charge. Although I don't know if this is still happening.