Is there a Post Office in or near the Rostock Hauptbahnhoff?
I was at the Rostock Hbf in 2012. Whether the Hbf has a post office such as Frankfurt Hbf. or it's just outside, I don't know. If you only want to buy some stamps, such 75 cents for postcards to the US, ask at the main book/magazine shop at Rostock Hbf whether stamps are sold there. At some stations (Hauptbahnhöfe) the clerk/cashier have stamps to sell.