In the Accounting/Billing area you will find all products that you have or had in production and have not yet been billed.

If an invoice is to be created automatically when an order is placed, you can set this in the product template. Simply activate the option "Create invoice automatically when order is placed" - then no post calculation has to be carried out and the invoice will be invoiced automatically.

Otherwise, each product must be recalculated before an invoice can be created. In this way you can directly view the additional costs incurred and optionally invoice the customer.

If you create a complete invoice, all products will be offered as they are and the complete print run will be placed on the invoice.
If you want to create an individual invoice, you can bill partial quantities or only certain products. 

​Of course, you can also add a free additional item as usual, e.g. to be able to calculate subsequent shipping costs or other additional services.