I don't understand this comment; my travel agent's credit card is never tied to my booking. (And if your bookings aren't showing up on your credit card statements, you should RUN, not walk, to a new travel agent.) A passenger may get a refund to the card on file for the account (his or her own), but one should also be able to get cash on the last night.
As dcassetta said, I was referring to the onboard credit, not the payment for the cruise. Payments made to your onboard account in advance of your cruise can be made by anyone who has your booking number (we occasionally gift OBC to nieces/nephews traveling with us ). That's the same way you get your onboard credit from your travel agent. They make a separate payment to DCL to gift you that credit.
The post I was responding to implied that if you didn't use that credit, it automatically came back to the passenger -- in cash. It doesn't - it goes back to whoever made the gift in the first place. So if you don't use your travel agent's onboard credit entirely while onboard, you lose it, the travel agent gets it back.
It's rare that it has happened (all credits would be used before they started charging the passenger's form of payment) but I do remember a post or two several years ago about individuals who were on a short cruise and didn't use their TA's OBC and were furious after the cruise to learn they didn't get the money from DCL and their travel agent wasn't giving it back to them either. Last year we were on a Hurricane Matthew and I had gifted my niece enough OBC to cover excursions for her and her husband that we had planned. I confirmed with DCL what happens with credits like this and from the travel agent once we knew we were rerouted since we wouldn't be taking any of the pricey excursions we had planned. I was assured that anything left from my gift would be refunded back to my credit card. We checked while onboard to make sure the travel agent's credit was used first!