THE ANSWER, PER USPS 2019 DOMESTIC MAIL MANUAL - THE USE IS ILLEGAL.
1.2.4 Paper or Cardstock
A card must be of uniform thickness and made of unfolded and uncreased paper or cardstock of approximately the quality and weight of a stamped card (i.e., a card available from USPS). A card must be formed either of one piece of paperor cardstock or of two pieces of paper permanently and uniformly bonded together. The stock used for a card may be of any color or surface that permits the legible printing of the address, postmark, and any required markings.
1.2.5 Acceptable Attachments
A card may bear an attachment that is the following:
a. A paper label, such as a wafer seal or decal affixed with permanent adhesive to the back side of the card, or within the message area on the address side (see 1.2.7), or to the left of the address block.
b. A label affixed with permanent adhesive for showing the delivery or return address.
c. A small reusable seal or decal prepared with pressure-sensitive and nonremovable adhesive that is intended to be removed from the first half of a double card and applied to the reply half.
1.2.6 Unacceptable Attachment
A card may not bear an attachment that is the following:
a. Other than paper.
b. Not totally adhered to the card surface.
c. An encumbrance to postal processing.https://pe.usps.com/cpim/ftp/manual...tandards.pdf
The post card has an unacceptable attachment.