It would not have been delivered. There is a limit to the amount that can be mailed by those prepaid deals and the post office has been aware of this trick you years so they just dispose of over heavy packages.
I've wondered though with the quality of modern printers and scanners and the simple text in a box noting that the envelope is prepaid if one wanted to they could make multiple copies of said envelope on to blanks and mail each part of the card back in a different envelope. But the cost of blank envelopes and ink would hurt you more than them.