Going from my past experience with making fabric post cards
what I use is "Peltex 72F" it has fusible on both sides, and was
easier (for me) to find in stores.
Also for postage to mail a FPC it was .20 more because of thickness,
and type of post card.
Unless the USPS has changed since the last time I mailed a FPC
which was about 3 years ago.