Yup, it's all a trap - forcing you to go into the store, or else one could just speed run through the mall corridors to get to the stairs each time.
There were originally going to be a lot of other things you can buy, but I only had 7 days to make the game, and so it ended up just being restore health, restore mojo, max health, and max mojo. If/when I get back to working on the game, there would be other effects like increasing the smite range, faster walking, faster smites, farther radius for picking up the coins, better discounts, other attacks/spells, outfits and accessories, etc.