dottie: Is this something that needs to be customized?
Nope. The invoice number's are being generated by QuickBooks so you'll need to wait for the sync to catch up after saving the invoice. When you create a new invoice and click save in Method, wait a few seconds then click the refresh button on the grid and you should see the invoice number populate. If you don't see that number then you'll want to check to make sure QuickBooks is open to the correct data file and the sync engine is running.
There's an alternative here which allows Method set your invoice numbers but that's only if Method is used to create all your invoices. You can't have Method and QuickBooks setting invoice numbers or you'll run into an issue. To change this setting click the QuickBooks tab then the Synchronize tab link - at the bottom of the page is the setting to assign ref number in Method.
~C