"Now there's Marvel Super Heroes Fun for Everyone!"
Mail in five 'proofs-of-purchase' from boxes of Trix, Lucky Charms, Cocoa Puffs, BooBerry, FrankenBerry or Count Chocula & you would receive two PrestoMagiX sets — one of Spider-Man, & one of the Incredible Hulk. The advert (this one is from 'Tales to Astonish' No.6, from May 1980) includes one 'proof-of-purchase' to encourage you to purchase more proofs.
The offer expired on February 10th 1981.
Not only could you send off for PrestoMagiX sets, but also, while that offer was running, you would find one of eight Marvel Super Heroes stickers inside the cereal boxes. The fact that this isn't mentioned in the comic advert implies that perhaps the promotions weren't entirely concurrent; I would guess the stickers came along after the PrestoMagiX offer had already started.
Original box photos courtesy of David Welch
This is an early use of Marvel's time-travelling shenanigans; the kid in this picture has clearly escaped from 1907.
Picture Credit: The SPLAT Scan Archives — David Welch
© Tom Vinelott 2020