I'm adding a payment feature to my website https://chat-tempmail.com, and I've chosen creem.io as the payment tool. While coding, I encountered an issue:

For example, my VIP costs $5, and SVIP costs $10. When upgrading from VIP to SVIP, the user should only pay the price difference of $5.

Should I:

1. Create a dedicated "price difference" product for this case?
2. Pass a fixed discount code when creating the order?
3. Or is there another approach to handle this?

Looking for the best practice to implement this logic.