I believe that you are remembering incorrectly. When only one ship is porting in Cape Liberty, everyone gets to park in the garage. This time period is roughly November to March.
Hmmmm. After a quadrillion cruises out of Cape Liberty, I was able to make my statement above. But I'll be boarding Liberty next month, and will see if that experience mirrors yours!
You are in luck! Anthem does not have a "real" Starbucks, so you can use the coffee card ANYWHERE AT ALL ON THE ANTHEM.
Enjoy!
(no advantage to buying in advance; same $31 before and during)