Joanne:
It sounds as if you are using the "Historical" option to look at the complete list of 7 years of invoices for all customers.

Click on the "Search" button just beside to the "Historical" button on the main screen. From the "Field" drop-down list, choose "Customer number". Then choose "Equal to" from the "Operation" drop-down list and enter the Customer code. Click on the "Search" button and you will see all Invoices and Credit Notes for that specific Customer. You can go to the bottom of the list by pressing the [End] key on the keyboard.

While you are there, look at the other search options available. The "Search" function is an underused capability in Invoices.