Amazon Pay by Invoice allows eligible Amazon Business customers to purchase items on credit and receive billing invoices for their orders. Amazon Business customers who have access to Pay by Invoice have terms such as net 30 that determine when their invoices are due (30 days after bill date in case of net 30).
Pay by Invoice will only show up for sellers participating in Amazon's business to business (B2B) program. If you have had a B2B sale you can see the settlements by going to Reports > Payments and then selecting Invoiced Orders.
These settlements will usually be smaller and not be on the same schedule as your normal settlements. You should have received a bank deposit to reconcile with for the same amount listed on these settlements.