As the user moves from record to record in the subform, the records are automatically saved.
If you have specified Cascade Delete Related Records for the relationship between the main (orders) and sub (line items) tables, deleting the record in the main form will automatically delete all related records in the subform.
The alternative would be to use unbound forms, but that is a lot of work.
Again, if Cascade Delete Related Records has been set, deleting the record in the main form will automatically delete all related records in the subform. Depending on the way your inventory system is set up, you may have to add the items to be canceled to the inventory before deleting the record in the main form.
Thanks for the usual clear explanations. I'm going to test this out over the next couple of days to make sure that I'm set up correctly to handle these scenarios. I'll let you know how it goes either way.