Thank you for the tip. I already did the first part and it's working! :)
Both in 'Continuous Form' and 'Single Form'. The text box I set up in my subform's footer 'Sum of Extended Totals' is calculating the sum of the line items - which is the first step...However, it's not doing my end goal yet, which is displaying it on the main form.
In your reply you said 'If you wish to display the Sum in the Main Form, you must set the criteria in the fields where you select items and quantity for a Purchase Order', can you please tell me what you mean by that? Thanks a lot!