You can make payments on open invoices in the Customer Center.
Click on "Make a Payment" in the "Customer Center - Home Links" section. (see screenshot below)
Once on the "Payment" page, you can select which invoices you would like to make a payment towards. Simply check the "apply" box next to each invoice you want to include. (see screenshot below)
Next scroll down to the "Enter Credit Card Information" section. This is where you will enter your credit card number. You can also select whether to make this the new default credit card on the account and save it on file.
Click "Submit" and you are done! Your invoices have been paid.