@Elle_Bi
The coffee card is one of very few things that never gets discounted and is always the same price before the cruise and on the cruise.
As others have advised, wait until you are on board.
Then go to the coffee counter (it's called La Patisserie on the Odyssey) and ask how many punches they will make for whichever kind of drink you prefer. If it's only one, buy the card. If it's two (or more! 😮 ), don't buy the card and just pay as you go.
You can use the card for anyone. You could get in line and yell out, "Coffee is on me!" and treat everyone in the line, if you wanted to. 😄