Keep half of that cool card someone sent you! The side without writing on the back can be easily repurposed into a postcard. If there’s writing on the back of both sides, trim the card into smaller squares or rectangles, slather some glue on it and paste it to a blank card (aka a sheet of heavier weight cardstock folded in half) and you have a new card.
Grab a a plain envelope to fit it at the stationary store (or get creative and make your own) and you’re in snail mail business.