I talk about this in “Escape” (did you download your copy yet?) and on this site. What works best depends on where you are and, apparently, the phase of the moon or some such abstraction. This is just an update on my experience in Guatemala.
I have an assortment of “used to work” experience. I think the main problem is that the company on the receiving end is not the same company as on the sending end. I know this was the case with TransExpress which worked perfectly in Costa Rica but sucked in Nicaragua.
I am now using AeroPost (https://myaccount.aeropost.com/) here in Guatemala. If was previously recommended by a few people but some other options were cheaper. My experience has been excellent. Packages get delivered in a timely manner but, most important, is the visibility. Their web site (like all of them) allows you to pre-alert packages and you then get visibility as to the package status. Yeah, they all claim they offer this but their site actually works.