The shop only has a few items for sale each time, so after it runs out of them, more items are unlocked later as you progress through the game (e.g., as you gain more stars).
Glad you enjoyed both this game and The Ghost Cage! You might like Forest Star as well - the gameplay is a little different but it also has multiple endings. It's available for free on Itch.io: https://seaphoenix.itch.io/forest-star.