Here's a photo of the postcard I received (found this thread on Google, so I hope someone's still paying attention to it). Sorry about the poor quality, if needed I can scan it to get a better version. It's got official logos from Carnival and US Airways on it, but the text you can't read on the bottom reads:
Certain restrictions apply. Call for details of participation. *This promotion is not sponsored by or affiliated with Carnival Cruise Lines, but they are a major supplier. Recipient is responsible for any applicable deposits, port charges, taxes and/or fees.
I'd be willing to pay the fees, we've already been planning to take a cruise so even if we had to pay a couple hundred in total it'd be a better deal than paying full price. I just don't want to get caught up in something that's more trouble than it's worth, and this strikes me as likely to be a very annoying ordeal potentially.
I'm planning on calling them from a service I can block their number from if needed. If someone else intends to call them, then I'll hold off and wait for a report.