I have had great luck just using the national service. Depending on how big/how heavy the box, it's been pretty cheap and extremely reliable. I've never had a package get lost, knock on wood! Also, it's usually only a few peso more to send it certified.
I have sent my labrador with them from the Bahamas to Buenos Aires via Miami but they go everywhere.... there were fantastic!!! and cheap it was either 4000 dollars with an other company or 600 with them, and they took care of my baby like a king they even sent me pictures of him while in Miami etc etc!!!!
Luis Alaez Global Freight Systems, Inc. 6310 NW 99 Ave. Doral, FL 33178 Phone: 305-513-9472 Fax: 305-470-6201 www.gfsimport-export.com