You can bring up these records in a found set and then loop through them dupicating each line item record and assigning the InvoiceID of the new invoice to each newly duplicated record so that it appears in the invoice's line items portal. Basically, you are duplicating a customer's invoice, both the invoice and the listed lien item recors.
Here's a thread that demonstrates this technique. It duplicates a bill of materials, but the concept and script steps are the same:
Dear Phil, I am quite new at Filemaker.
Is FoundCount a global field in BOM?
Thanks in advance.
No, it is a parameter passed to the Get function. Get functions are very useful in FileMaker. You can look them up in FileMaker help to learn more.