I would second the idea about custom-designed products in most cases, but not in this one. Unless you can draw yourself (or hire the talent from another source) it'll be difficult to sell custom-made items in the comic book niche.
Maybe make a money site with affiliate links to comic books and memorabilia? Age-up that site for a bit then hit your Instagram followers with a post every time you come out with a new product, in-between the more organic posts. That should generate something, but you'll probably need to test a few times before you get down exactly what to say and exactly what to link to.