Are you a customer that has an open credit to apply?
At the top of the "Pay Invoices" page, you may have a button that says "Apply Credit". Click this button to select the credit you wish to apply, and the invoice you want the credit applied to.
PLEASE NOTE: At this time, only some companies that use Fidesic allow for applying your credits through the site. If you have unapplied credits and do NOT see this option, you will need to contact the company directly to apply credits to invoices.