If a Guest selects the pay later option during their purchase, the Events Grid will show a Dollar Icon on the left side of the row. This Dollar Icon means that there is money owed on that purchase. 

To determine if there is an open invoice for the transaction, click on the Dollar Icon. If there IS an invoice related to the transaction, you will see a blue highlighted message and link at the top of the window to pay via the invoice.

If no message appears on the window, you can select which items you are adding to the payment. Once you have selected the items the total will be displayed and you can enter the Payment Type, Check Number if applicable and a Memo.