The post office for both Ixtapa and Zihua is in downtown Zihua, near Pollo Feliz chicken house and across from Funerales del Pacifico funeral parlor. It's open weekdays 8-3 and Saturday 9-1. You can ship packages through the government-run MexPost, which is right on the premises.
Although many hotels in Ixtapa and Zihua have business centers with Internet access, cybercafés are where most computerless Mexicans go to do their e-mailing. They are inexpensive -- about $1 an hour -- and ubiquitous, although harder to find in Ixtapa than Zihua.
El Navigante Internet (Calle N. Bravo 41, between Calles V. Guerreo and Galeana, Zihuatanejo. 755/554-0544). Ixtapa Conexión (Plaza Ixtapa, Paseo del Palmar s/n, Ixtapa. 755/553-2253). Net World 2000 (Calle Juan N. Alvarez No. 34. 755/554-2956).
Correos and MexPost (Av. Correos esq. Calle Telgrafitos. 755/554-2192).