Germany - Frankfurt Oder
I visited Frankfurt Oder - an hour by train east of Berlin. It is at the border of Poland with the Sublice town of Poland on the opposite bank of Oder River. There was no postcard sold in Sublice, Poland and so I bought a Polish stamp from the local Polish post office and put on this postcard which shows Frankfurt Oder in the foreground and Sublice in the background. And put the Poland stamp on the postcard and mailed to myself.