If you have multiple sales orders from a customer that you want to group into a single invoice, you can do this from the Invoicing module.
1. Open the Deliveries tab in the Invoicing module. You will now get an overview of all executed deliveries that are already in the system but not invoiced yet, grouped by customer.
2. Check the deliveries that you want to invoice and click on the list action "Create grouped invoice (per customer)" at the top.
3. One draft invoice will now be created, grouping all these deliveries together.