Post office
Unfortunately, most of the post office employees don't speak English, so be prepared when going to the post office.
Closest post office
Masarykova Dorms
Kafkova 102/19, Praha 6 – DejviceStrahov Dorms
Břevnovská 1692/6, Praha 6 – BřevnovReceiving packages
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/letter
Affix there a stamp marked "E" (CZK 48) for postage within Europe or "Z" (CZK 54) for postage outside of Europe. For postage inside the Czech Republic affix a stamp marked "A" (CZK 36).
Find an orange post office letter box:

You can find more information at the website of the Czech Post