Generate invoices for a membership site. How to handle the advanced payment

Generate invoices for a membership site. How to handle the advanced payment

Hallo,
I'm try to handle this scenario:
- I have a membership site where readers can access after they pay the subscription feed with PayPal. This is handled by my membership software.
- Once a week I import the new customer data in Zoho Invoice and send the invoices (in the future I'll do that with API, but it's another story)
- Since the client has a already sent his payment I want to make it clear and evident on the invoice.

The only way I can think is to add a Custom Payment Term that says something like "Already payed"
Any other way?

Thanks
Stefano