All partners have the ability to pay their bills from within the management portal. The directions for doing so are as follows...
Log into the management portal, go to the Pay Bill tab and select View/Estimate Bill.
Choose an invoice to pay by clicking in the white-space on that invoice's row. Then, hit Pay Invoice.
In the window which appears, fill out the credit card information required; if you have stored payment information, it should auto-fill these fields.
When all the necessary fields are filled, click on Make Payment.