After looking up your invoice:
If you are signed in as a member, you can click on the Invoice number to view and print the details of an invoice. To select one or more invoices to pay online, click on the checkboxes. The amount charged to your credit card when you click the "Enter Payment" button will be the sum of the Balance Due amounts on all the invoices that you select. The purchase amount column identifies the full invoice amount.
If you are paying on behalf of a member, click on the line once and within a minute the invoice will pop up below. From there you can print or download the invoice prior to paying.