Take a numbered ticket (if you just need to mail something, it's N°1 you want) for the services you require and don't miss your turn. 

Also at A-4, Praha 5, Újezd 15, tel. 257 32 27 92, open 08:00 - 18:00. Closed Sat, Sun; D-2, Praha 1, Kaprova 12, tel. 224 81 15 87, open 08:00 - 19:00. Closed Sat, Sun & J-3, Praha 1, Hybernská 15, tel. 224 21 97 14, open 13:00 - 24:00.