Check out the Invoice starter solution that comes with filemaker 11 for a decent example of how to set this up.
Generally you have at least 3 tables:
Invoices are one record per sales transaction. Lineitems list individual items, quantities, prices etc. in a portal on the Invoice layout. Products serves as a list of all products available for sale and their unit prices to be looked up into LineItems as the invoice is filled out. Products can also be used to help compute current inventory levels.
With this approach, if your LineItems portal does not have a scroll bar, you can just print the same invoice layout used for data entry. If you use a scroll bar on the portal, you may want to use a summary report based on the Line Items table to print out an invoice as this enables you to have multi-page invoices.
If you want to use the summary report approach, here's a tutorial that explains how to set that up as well as several other useful reports that can be created for an invoicing system:
I wouldn't recommend FM starter solutions (why start people off on the wrong foot)? There are excellent demo files out there, such as the basic invoices demo by Comment here:
It includes a basic summary report as well.