Estafeta is a mexican courier service that has been in the market for quite a long time. They used to concentrate on the domestic courier service (most of it by land) and had partnerships with international courier services for international packages. They are the leading domestic courier service in Mexico covering almost every city.
The new product of Estafeta is "Estafeta Carga Aérea". They started flying very small planes one or two years ago, and for 2002 they leased three B732C's to handle the air cargo and started flying to the United States.
To answer your questions:
They operate flight to many Mexican Airports. The most common airports served are: MEX, SLP, TIJ, CDJ, MID, HMO, MZT, CUU and MTY.
They started operations to MIA and will increase destinations in the USA in the following months.
For the moment they will stick to the B732C, since it is the new adquisition, but we can see in the future some B727C or even bigger airliners.
As Thomas_Jeager said, the word estafeta in spanish can be translated as courier, but has other meanings.