Typical invoicing systems are set up like this (but your names may differ):
Invoices----<LineItems(also called InvoiceData)>-----Products|Services
And the unit price is typically specified in Products|Services.
If so, then you should use either of the following two auto-enter field options to copy over the unit price from Products|Services into a number field in LineItems each time you select a Product|ServiceID in the portal to LineItems on your Invoices layout:
Looked up Value
Either method can copy over the value. And it will not automatically copy over a new value if you edit the matching Products|Services record to change the unit price for that item. Thus, the computed invoice totals for pre-existing invoices will not change due to a subsequent price change.
You can open the Invoices starter solutions for FileMaker 11, 12 or 13 to see a working example of this method.
Your advice as usual has proven invaluable