I'm a little confused, unless there's something I'm not seeing.
If you're flying with a just couple of suitcases, why would you need a customs broker in Mexico or in Belize for that matter? Computers and some peripherals aren't even subject to duty, at least not in Belize, nor are most personal items that you'd carry on a plane. Even if you believe you plan to stay in Belize (plans can and often do change), items for personal use as a tourist aren't subject to duty.
If for some reason you want to bring in a bunch of stuff on the flight, your excess baggage charges will be huge. Anything over 50 pounds is going to eat you alive with excess baggage charges.
And why drive into Mexico to fly to Cancún? I don't see the logic in bringing Mexican border customs into the picture if you're leaving from the USA and going to Belize.
If you want to fly to Cancún, there are cheap nonstop or at least direct flights from just about everywhere in the USA to Cancún. While there is a Tropic flight from Cancún to Belize City (BZE) and then another flight to San Pedro, that will likely cost as much, or possibly even more, as your flight to Cancún from the USA. You'd be better off either flying direct to BZE from wherever you live in the USA, or else fly to Cancún and bus to Chetumal on ADO and take a water taxi to San Pedro, or bus from Cancún to Belize City and then hop over to San Pedro on the water taxi or quick flight.
Have you researched this at all?
Or maybe I'm missing something?