It seems that people have had different experiences at terminal #5, but as someone who used that terminal 3 times in May, and twice in December, I always pay as I entered the garage. And that's the fact, by paying up front on a B2B, I only paid for the first cruise, and a "freebie" on the second half. That is NOT the case for other terminals.
After looking at my sign/sail cards, that was late April, not May. Sorry about the time period.