On our Holy Land cruise we had coupon books that had a coupon for the coffee cards: buy one get one. So three couples bought coffee cards and split the cost with three other couples. Worked out great.
It also meant we had more than enough for the trip and several of the group finished the cruise with cards not fully punched. Knowing we would be sailing Princess in January, they gave us their cards.
We had absolutely no problem using the coffee cards on our Panama Canal sailing. We used them the entire cruise and finished off the last punches getting cappuccinos on disembarkation morning.