Sandwich them between stiff cardboard so they don't get mashed up and ruined in the mail. Post clerks love to bend stuff, fold stuff, and generally wreck it. I'd see about holding the stickers still in the middle of the card, so they don't slide with all the shaking they're bound to get, too. Maybe put them in little plastic bags, and stick those down to the card? Then surround with more card!
Remember you're up against these guys:
![Image](http://static1.squarespace.com/static/521bb31ce4b0f506a31e4deb/52692387e4b0a6ea6c32f102/526923b8e4b00e1ae8e4dfa9/1382622138332/Rickshaw+Run+Highlight+Reel.jpg)
Royal Mail grade care. Roads and all. They have smashed Model M caps for me in the past. Smashed!!!