Looks like the problem is AA.com is not letting you buy OW tickets. Only RT permitted.
From taxes and fees breakdown, I'd surmise only Cuba departure tax is permitted. Good question where you buy the tourist card!
US APHIS USER FEE (UNITED STATES) $3.96 USD
TRANSPORTATION TAX (UNITED STATES) $35.60 USD
PASSENGER FACILITY CHARGE (UNITED STATES) $13.50 USD
US FEDERAL INSPECTION FEE (UNITED STATES) $7.00 USD
US SECURITY FEE (UNITED STATES) $11.20 USD
US CUSTOMS USER FEE (UNITED STATES) $5.50 USD
OTHER TAXES $25.00 USD