1. Every user has an unique address.
2. Check balance of the address. If balance < copper price: missing (copper price - balance).
3. It’s probably a script on the forum’s backend. It’s not hard. You can make your wallet call a function when receiving a transaction, which will compare the amount with the fixed price of the copper upgrade. If it’s equal or higher, sets your account rank to Copper Member.
You could probably use something like WalletNotify for that:
https://github.com/damonp/walletnotify