El Correo, the country's postal service, is improving all the time, and is fine for postcards and letters. To ship important packages, use DHL, FedEx, or UPS. All have offices in the New City.
UPS (12 C. 5-53, Zona 10. 2421-6000) DHL (12 C. 5-12, Zona 10. 2339-8400) FedEx (14 Av. 7-12, Bodega 20, Zona 14. 2366-8536)
El Correo (7 Av. 12-11, Zona 1. 2232-6101)