Depending where you are in Mexico there will be different challenges in sending and receiving mail and packages. There are private mail service companies available in most cities that will help foreigners. The best thing to do is to ask someone in the city that you will be staying in to give you a recommendation.
Here are some links to the most popular mail services.
Mexico Post office (Correos de Mexico) – This is the regular mail company in Mexico. This service is slow; a letter can take up to 2 weeks to travel from the US to southern Mexico. For more information go their website: http://www.sepomex.gob.mx
Mexpost: This is the Mexican postal system's version of express mail service and it belongs to “Correos de Mexico”. This service is provided in most post offices in Mexico. They offer a delivery time of 3 working days in foreign orders, and next day service within Mexico.
US Postal Express Mail: Is delivered here by MexPost. Service is excellent. They deliver.
FedEx – FedEx Freight covers every major Mexican market, here is FedEx website: http://www.fedex.com/mx
Estafeta – Delivery of documents to anywhere in the United States and Canada within 3 to 5 days. They offer a delivery time of 3 working days for foreign orders, and next day service within Mexico. Only documents are allowed if you are sending to a Post Office Box. If you need further information check their website http://www.estafeta.com
DHL – This Company provides express mail and packaging services in Mexico and around the world http://www.dhl.com.mx
Multipack - This is another express mail and packaging company working around Mexico with international services as well. http://multipack.com.mx