Touch a part to open a GUI shop

You can use Region3 funcion 'cause, it’s complicated to use TouchEnded Event, so, if you use Region3, you need check if player is in that region. If you wanna use the best form to use Region3, you should to use ZonePlus, 'cause is the best way to use Region3 without a lot of codes.

ZonePlus: ZonePlus v3.2.0 | Construct dynamic zones and effectively determine players and parts within their boundaries - Resources / Community Resources - DevForum | Roblox