Hi there,
Glad to have you join the Community. I'll be glad to guide you in the right direction so you're on the right track for achieving your goals.
In order to add the percentage fee to the invoice, I recommend adding an additional line item with the manually calculated percentage fee. This line item can be added when creating an invoice. Otherwise, if you're looking to auto add a percentage fee, I recommend looking into third-party apps that can help you with achieving this goal. You can search for apps on our site www.apps.com.
Let me know if this info helps. I'll be a message away in case you need anything.
