Looks like you have repeating fields here. First step is to get rid of them. For a typical invoicing solution, use a portal to a related table of "line items"--also called "invoice data" in the starter solutions.
This allows you to add rows in the portal as needed--which adds records in a related table.
To print the invoice, you then bring up a found set of these portal records on a list view layout with fields from the Invoice record included as needed in header, footer and/or grand summary layout parts.
The typical data model--which is used in the invoice starter solution, but with different field names that comes with FileMaker looks like this:
Invoice::__pkInvoiceID = InvoiceData::_fkInvoiceID
Products::__pkProductID = InvoiceData::_fkProductID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained