How have you designed your invoice?
Invoices usually take at least three tables to work the way you need:
Invoices, LineItems, Products
You place a portal to LineItems on your invoice layout and line items looks up price and other product info from the Products table.
Is this how you've set up your database? If your portal is a nonscrolling portal so that all items purchased are always visible in the portal, you can simply print this layout. If you need a scrolling portal to allow customers to purchase more items than can be listed in the portal, you can set up a summary report based in the LineItems layout and use it to print your invoice.
Here's a tutorial on Summary Reports that does exactly that:
Creating Filemaker Pro summary reports--Tutorial