I think selecting subform records has big issues. Try using a multi-select Listbox instead. The items selected in the listbox could be the criteria for a SELECT aggregate query to return the sum of the selected products costs. Will require VBA code to construct the SQL statement. Check this
http://access.mvps.org/access/forms/frm0007.htm
The VBA could populate a textbox on form with the sum. User enters the other info.
How many products would user have to choose from?
What is purpose of the form? Enter new product or new order?