If you take meat, cheese etc. into Belize it's always a good idea not to repackage anything to save space. They look for USDA labels if they open the cooler.. If you leave with seafood it's always a good idea to have a bill of sale or something equivalent to show customs in the US.
Years ago when the Co-op was a viable concern, a receipt from them was good as gold...
I couldn't imaging leaving without some snapper filets or some conch...
Also, if you're a beer drinker, the beer in Belize is wonderful.. Have the cab from the airport make a stop at the Belikin distributor and pick up a case or two of the short bottles.
Last price I remember was about $22.00 US a case which is a real savings down the road..