It is a goods tax free zone for non-Belizeans that do not reside (or are on vacation) in Belize. This a to stimulate foreign trade and receive foreign currency.

If you shop in the free trade zone you should have to enter and exit via Mexico. When you renter Belize you would be expected to pay import taxes, gst and/or stamp duty on the items you bring into the country.

And departure taxes would be applicable.

That is the theory - I'm not sure exactly what happens day-to-day.