Think of MARVEL SUPER HEROES SECRET WARS and you probably think of Jim Shooter's year-long slam-jam (and the UK reprints). Maybe you think of the Mattel toys that kick-started the whole thing. But, it's easy to forget that Marvel and Mattel turned the whole thing into a multi-partner licensing extravaganza.
Here's a 1985 US advert (lifted from an August cover-dated Marvel monthly) from Dorman's Cheese hawking a free set of Secret Wars puffy stickers and associated album (not to be confused with the British sticker album I've looked at in a previous post).
AFAIK, puffy stickers are those foam-like 3D stickers. Not embossed or anything clever like that... just fat stickers. I very much doubt these were exclusive from the cheese people although I couldn't say whether they reached British retailers.