You can purchase it with gift cards. It gets rolled into the total cost of the cruise if you add it on before the PIF date.
I have found through American Express better coverage for a lesser cost than what
DCL sells. I have not had to use the coverage, so I don't know how good it is, but just throwing that out there.
One disadvantage the gift cards have with cruises if if you end up cancelling, they will refund to the original payment. For our recent November cruise we were keeping a massive pile of gift cards in the house "just in case", as the $25 cards were the most affordable inventory at the time. Had we needed a refund, we would have had all these $25 gift cards.