I'm fairly new to Filemaker. I'm trying to set up a basic sales order & invoicing system. This part I have so far works, 4 tables:

Customers

Products

SalesOrders

OrderLines

SalesOrders relates to OrderLines via a unique SO serial number, and the SO-OrderLines portal works fine. But I'm stuck trying to set up the invoicing. Problem is, each sales order may have multiple line items, and all line items may not ship at the same time. So 1 sales order may link to multiple invoices. I don't understand how to set up the relationships between SalesOrders and Invoices, and Invoices and OrderLines. Do I need a separate InvoiceLines table?

It depends - does a single line item always ship in its entirety, or is it possible to split it into multiple shipments?

I can get by without splitting line items. I'd like to keep it simple and require line items to ship completely.

Then I see no reason why you couldn't simply "share" the line items between Orders and Invoices. The relationship between Invoices and LineItems would be very much like the one between Orders and LineItems, except you would assign existing LineItems to an Invoice, instead of creating new ones in a portal.