From 1983: MARVEL UK loved to give away paper-based products as freebies. Not only did they cost next-to-nothing to make but they also avoided the hassle of having to employ someone to tape them to the cover.
Case-in-point: the second and third issues of the short-lived 1983 X-MEN weekly (plagued by Marvel's new Crappo Colour technology) came with free loosely-inserted X-stickers. These, of course, were guaranteed to part company with the comic when displayed vertically on the shelves... forcing the would-be buyer to first vigorously shake the comic and then, upon discovering its absence, scrabble around the shelves to try and find it. This could also, depending on the display, unveil a treasure-trove of other gifts and inserts which had long-since parted company from the comics they were intended for. And, of course, unscrupulous buyers could slide inserts from other copies into their own and walk away from the till with multiple inserts tucked inside.
Marvel went for economies of scale by getting several batches of stickers made at once... around the same time THOR (which launched two weeks ahead of THE X-MEN), the revived MWOM, SPIDER-MAN and possibly others all boasted free stickers.