Good question. A true GPC (Government Postcard) has the postage printed on the card itself. You would purchase them from the post office for the price printed on the card. You could then mail that in an envelope to the person. They could sign it and drop in the mail where it would be sent back to you. There are also blank postcards with stamps affixed to them which are not true GPC's. Once it is cancelled with a machine or hand cancel it can no longer be used for postage. I am sure one could slip through the system. If you were mailing a postcard from Spokane, WA to Mobile, AL and it was stamped with a Mobile, AL cancellation the Post Office would reject it. When there was hand sorting of the mail there is a greater chance they would reject it and return to the sender if they had the address. If not, they could forward to the addressee, stamp it with a "Postage Due" stamp and collect the postage on the receiving end.
Here are a couple of them. The top one should have been cancelled when it was mailed from California in the late 1940's early 1950's, but it slipped through the cracks. It has a legitimate signature on the other side.
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