Create a membership verification system

Hello! I am a manager at a small, locally owned coffee shop and we are trying to launch a membership club. There are two levels, Gold and Diamond. We are using squarespace for the member registration and payments. We use toast for in-store purchases and Square for online orders. Both POS systems have a camera that would be able to scan a barcode or QR code. Our system right now is members are sent a custom keychain (no QR code on it, just our logo) and are told to show their keychain at the register to confirm they are members. The problem is we need to be able to assure that people who show a keychain are still subscribed. They could still have the keychain but aren't paying anymore. We don't want to have to look up each member from a list everytime they want to order a coffee. Any ideas on how to integrate some sort of verification system? Thank you!