Need to have all lines on a sales order interface to AR as 1 invoice

We are on 11.5.10.2

I need to put inventory items and a non-inventory item on a sales order and have them go through AutoInvoice and create only 1 invoice.

Example:

SO 1234 has  line 1.1 with item 420000 with qty of 15 at $2.00 - this is a true inventory item that will ship. Ship set 1

                      line 2.1 with item 50009 with qty of 1 @ $0.0 - this is a memo line that needs to appear on the invoice. Ship set 1

After shippingt and autoinvoice I get 2 invoices for SO 1234. One for line 1.1 and another one for line 2.1.

I need a single invoice with both items listed.

What set up do I need to change? I've read about the fulfillment sets and it is greyed out on the sales order form. How do I get that field to open up?