For shipping anywhere I've lived or sailed its always best to "go local"....otherwise your odds of having problems are much higher. In most W Caribe countries FEDEX etc just deliver to a local agent...and that's where the problems start...had documents FEDEX'd to me in Belize
once that took 2 weeks to get to me AFTER arriving in Belize
In Guatemala, I've used TransCargo a lot (Captain John is a rep for them). Their customer service is not always great, but they get it there at a very competative flat rate (no additional charges for Customs etc) via surface transport (ship, so it takes a while). For lightwieght/small stuff I use CPXBOX. They provide shared air freight services so are much quicker, but more expensive (still radically less than FEDEX etc). Excellent customer service, but they are by the book so you pay the full monty on all duties.
In Panama, Marine Warehouse is hard to beat if you've got the time. They send down a container once a month by surface transport. Best to pickup in Colon if you can because in-country transport can cost as much or more than the original shipping (sending to Bocas for example). I've also used AirBox Express, similar to CPX above, with good results. They provide surface transport services too, but I've not used those yet. In Bocas del Toro, MBE provides excellent, but expensive, service.
In Belize, you need what Belizeans call an "ally" in Customs or know someone who does. Without such, you will get royally screwed and inconvenienced.
My experience with Marine Warehouse has also been good and their prices are competative.