Instrucciones en inglés:
So, you should go to https://www.fratec.net/ and log in using the username and password you got via email.
The first option on the menu is “Facturación”, there you will see all your invoices, paid or not.
If it’s paid, there will be a small Icon on the right and if you hover over it, it will say how and when it was paid.
If it’s not paid it will have a check box on the left where you can select as many invoices as you want and by clicking "Pagar / Reportar" it will send you to a page that has the “PayPal check out” button.
Try it out, if you have any questions let me know.