Departure Taxes
Are usually in your ticket except some charter flights or tickets issued before the taxes went up. I had to give AA $10 at the ticket counter in July for a ticket bought before the increase.
Also for charters they often give you the tourist card when you check in in your country so you may not have to buy one at all.
If you are on AA, you can buy them ahead of time at the international/Caribbean check in area (in MIA anyway) or if you are a member, in the Admiral's Club.
Better that you have it before you arrive as it takes a bit of time to fill in.
