Originally Posted by alien

Pedro's over Martha's.

It's about a 15 minute walk from the airport....actually, it is partially adjacent to the east side of the airstrip. If you could walk down the strip it would be less than 5 minutes. It is very easy to get to. From where the ferry docks, it's a further 5 minutes walk away.
If you take a taxi, which will in all probability be one of quite a few rather old Toyota vans, it should be about $10 BZE from either the dock or the airport.
Pedro, aka Peter is a British expat who knows absolutely everything about Ambergirs Caye...great source for info...
I assume you've seen his website...http://backpackersbelize.com/
(If you click on "PHOTOS" at the top, Peter is in the first top left pix.)

I also assume your reference to a 3:30 arrival and staying the night on Caye Caulker implies you are taking the water taxi over? If not and you are going by air, I can assure you there will be others on the flight from DFW going to San Pedro that same day and Tropic Air (can't say about Maya) will get you there that afternoon...