I also use muslin for the address side of the postcard, so my postcard is all fabric. I use the pigma micron pen Judith mentioned. I take the postcard to the post office & ask them to please hand stamp it. They don't weigh it, because I have yet to put embellishments on it. I've been making postcards for a number of years. Don't think I'm an expert, but my daughter once said to me she'd like something in her mailbox besides bills. So she's been the recipient of all my postcards. Never lost one -- knock on wood. The stamp stays on!
That's my story & I'm sticking to it!