This is a series of ten Marvel soft-cover chapter books for kids. These books were only available through the "direct-to-home" Scholastic book club that you could enter by signing up through your school. That makes these books very hard to find... and it makes the associated "free stuff" even harder to find!
Each month's delivery included a set of trading cards (50 for the first month, 20 each month thereafter), a collector's card tin, a grow-your-own Spider-Man figurine, a sheet of stickers, and a collectible poster. All ten posters combine to form a gigantic Marvel scene with all the Marvel Super Heroes and Super Villains!
I thought I also read somewhere that there was a Marvel Magazine included each month. But I can no longer find that reference, so maybe I imagined it.