I have enjoyed making several postcards and sending them (none so beautiful as yours though!). The only information I have to offer is that the postage required on these is not the same from postal clerk to postal clerk. It may be more economical, depending on how thick, lumpy, rigid, etc. to send them in envelopes, which would also give protection. But then you have a greeting card instead of a postcard.
Have fun making them.
P.S. I make mine using that very heavy fusible Pellon interfacing and using a zigzag rotary cutter for the outer edge combined with a row of stitching an eighth or quarter inch inside the edge. Fabric is on both sides.