there are a couple pretty good books on fabric postcards.
a number of years ago (like 7 or8) i participated in a postcard swap- what great fun!
we did 4 seasons...i have a collection of wonderful postcards that came to me through the mail...all made of fabric- they are great.
we did not use fusable fleece- we used a stiff stablizer- (not everyone did them the same- some used regular batting with a fusable web...
we used muslin on the back (there are actually pre-printed muslin post card backs on the market-they have the address lines- and a dividing line between address and message areas.
they have to be hand cancelled at the post office- can not go through the machine.
i used timtex in mine (the same stablizer used in the rims of baseball caps)
embellishments were the only area we found a problem with post office- we could not mail ones that had beads, embellishments that really (stuck up)off the surface--thread embellishing was not a problem. the ones that could not be mailed as post cards we put in an envelope and sent- but 98% of the 34 i received went through the mail as a post card...with (1) first class stamp.