I'm really surprised you could book through because even Mexicana won't allow Americans to buy tickets to HAV onliine. You have to do it in person and not in the US. Unless you are under a special exemption, such as family visit, education or media junket, you are presumed to be spending money in Cuba the minute you land and violating. Yes, there is the airport fee of CUC 25, which translates into about $30 at the bloated official exchange rate, that is built into the price of the ticket, at least when you go via Cubana. (If you want a real aviation treat, try taking one of their YAK 43s.)