Thank you very much for your advices, I got it working now!
I made some improvements - the read only OrderDetails form is based on a query that lists each product once along with quantity and total price, the data entry form (shown upon adding new order or editing existing one) then shows each unit by serial number. I really like it, see the pictures:
Now I am trying to figure out how to DELETE a record in the subform, is there a way to add a button or something? If I try to manually delete it, it says that a product needs to be selected.
Thank you!
Tomas