Do you mean the border at Benque Viejo del Carmen? To my best recollection there are no duty free shops there.
On the Guatemala side, this is a third world border crossing...pigs and chickens roaming around, pickup trucks with 20 people hanging on to them, vehicles parked in the middle of the road, ancient busses belching oily smoke etc.
The Belize side has a nice new building - with clean washrooms
- for customs but I think that's it until you get into Benque.