Unfortunately, most of the post office employees don't speak english, so be prepared when going to the post office.
Closest post office
Masarykova DormsKafkova 19, Praha 6
Strahov DormsVaníčkova 1911/5, 160 17 Břevnov-Strahov
An invitation to pick up the shipment ("Výzva k vyzvednutí zásilky") is a paper which is sent to you at your dorm (mailbox).
It states when and at which post office you can pick up your package:
How to send a postcard/a letter
Affix there a stamp marked "E" (CZK 32) for postage abroad. For postage inside the Czech Republic affix a stamp marked "A" (CZK 16).
Find an orange post office letter box:
You can find more information at www.ceskaposta.cz/en