itch.io

Hi, no problem; I've just been making a menu for skills/inventory recently...

I'm slightly dyslexic so it's hard to explain (but it's very simple to make a shop/store) so if you want contact me (VP) on the discord servers, I can help you there - KeshaFilm keeps banning me from here, most of the pages are not accessible to me  and I can't normally reply....

You could use either the map_loop.script or an interactive (VN) script attached to the store (you could also build a store using the FSM, if you prefer).

You can use "buttons" or "hud images" to display items (you can also animate them and layer them).

I use $global.variables for the items (set them to 0 or 1 when you purchase them - or increment them with +1 to upgrade/power-up).
The FSM/scripts/loop.script can then reference those variables, to make use of the purchased items in the game.
eg: In my game, the flippers increase swimming speed of the player whilst equipped.