I don't quite follow what your screen shot is intended to represent.
The basic data model in the starter solution and in most invoicing systems built in relational databases is this:
The simplest approach--not what you'' find in the starter solution.
You set up a portal to LineItems (called Invoice data in the starter solution) on your Invoices layout and format the Match field to Products in your LineITems table with a value list of products and/or services you might list on an invoice.
The starter solution uses a much more complex system with a selection portal placed inside a slide control that is then placed inside a popover. This results in a more user friendly work flow as it's easier to find and select products or services for a given layout, but depends on a script to add records ot InvoiceData and enter a product ID value into the new record's match field in order to link that record to a record in the products table. Unfortunately, FileMaker Inc is really idiotic in that they release these starter solutions with nearly zero internal documentation and leave it to developers to puzzle out how the system is designed to work--which can be an irritating process.
Thank you for your response. While I chew on it to try and understand how to apply it to my fix ... let me make this simple observation of the Starter Solution "Invoices" ...
I see an "Invoice" tab, a "Customers" tab, and an "Inventory" tab. When I populate the "Inventory" table with three dummy products, why don't they appear in the "ITEM" field on the Invoice Layout - either by Dynamic populate or by Pull Down access?
How did you "populate" them? by typing in data or by importing from a file?
I do not see an Inventory tab. I do see a button in the Header named "Products". Is that what you mean by an "inventory tab"?
When I manually enter three records on the Product Details layout (Pressing the keyboard shortcut for new record on the Products layout automatically takes you to the Product Details layout.), I can then change over to the Invoice Details layout and find all three listed inside the selection portal that appears when I click "Add Line Item".