You can pay for it at the airport. .
1. On the plane, fill out the immigration form.
2. One you are off the plane, follow the herd thru customs. Get in line
3. As you walk thru the line, there is a tourist card booth, and you need to pay $10 USD. They give you a receipt and you need to present it at customs. It is a quick process.
4. Pick up your luggages and you are in paradise...