Updated for 7.0
On the left-hand side of the screen in the navigation bar, select My Account. This will take you here:
Click + Payment in the horizontal navigation bar. This will take you here:
If your billing address and card information are not already filled in, follow the prompts to add that information.
In the bottom right hand corner is a box next to a dollar sign with 0.00 in it.
In this box, edit the 0.00 to match the amount being paid for that particular invoice.
After editing the applied price, the payment, and processing fee, the total will appear below the box.
When done editing the payment, click the Submit Payment button below the Open Invoice.
FOR A MEMBER OTHER THAN YOURSELF
Select Invoices under ACCOUNTS from the left-hand side navigation bar.
Type in the invoice number on the right hand side of the screen in the search invoices search bar.
Select the invoice. This will take you here:
In the pop-up window, select the Edit button. This will take you here:
Select Edit within the Payment(s) box. It will now look like this:
Next to open payments, type in the amount being applied to the invoice.
When done, click the + button to save changes.