last did ~PCX52 and had a darkroom. I used ilford MGV and IV, RC, 5x7 for probably 10 exchanges with Ilford postcard paper before that. (I did bring it into the US PO before purchasing 100 sheets boxes, they said fine!) Remember the official US postcards were thin card stock (are?).
I put address, stamp, message on the back and dropped them in the post office. I did a lot of postcards, apart from the exchange, and never had a problem. I love the sometime marks or mangles, stamps and postmarks.
now that digital is possible I'll have to look into what i'll be doing. it probably be some sort of rc type paper.