From the Super Mario Wiki
Mario heads off to the Koopamart

The Koopamart is a store featured in the Mario comic stories published in the Nintendo Comics System. As its name suggests, it is a convenience store supposedly owned by Bowser (the building is even modeled to resemble the Koopa King), and Koopa Troopas work there as cashiers. Among the items sold there are Shy Guy Masks, G.I. Bullet Bill Heat-Seeker toy guns, and C. Mentum's Famous Sticky Juice. In spite of the fact that the store is owned by the Koopas, the Mushroom People are quite content to shop here. Toad is a regular customer himself, as he carries a Koopamart Kredit Kard.

The Koopamart is first mentioned by Toad in "Duh Stoopid Bomb!" Right when ditching his Shy Guy disguise to steal the bomb of the story's title, Toad points out to Bowser that he bought "this crummy robe" at the Koopamart yesterday.

The Koopamart is later seen in "Elect Mario for Man of the Year". Here, Toad lends his Koopamart Kredit Kard to Mario so he can pick up ingredients for his famous homemade pasta sauce. While there, Mario passes out pin buttons to the other customers and picks up a Shy Guy mask for a child named Elliot, who is having difficulty reaching it. Elliot's mom then tosses away the mask, which lands in Mario's shopping cart and sets the story's plot in motion.

Probably by coincidence, the Koopamart was also mentioned in the Super Mario World episode "Gopher Bash". Right when he's getting away with the stolen crops, Cheatsy Koopa taunts the Mario Bros., telling them "The only way you guys are gonna eat is by shopping at the Koopamart!"

In the Nintendo Adventure Book Leaping Lizards, Mario will mention that Princess Toadstool spotted Iggy Koopa with the magic silver whistle they need to change the Mushroom King back to normal while shopping for snails at the "Koopermarket".