packing Slip Query

I am trying to build a query that I can use to support a packing slip report.  I have a table (t-PackingSlip) that contains a packing slip number (increment), and a packing slip detail table [t-PackingSlipDetail] that contains specific shipment records. The relationsip between the two is one to many.

When a user creates a packing slip, they are prompted to enter an order ID number into a form for t-PackingSlip.  This action creates the packing slip ID number.  Then, they enter specific data into a sub form for t-PackingSlipDetail.

I would like to create a query that shows the following columns of data:  Original order qty., previous shipped qty., ship qty., and outstanding qty..  I think I've done a good job struturing my tables/forms/sub-forms to accommodate this calculation, but I can not quite figure out how to get started w/ the expression for the "previously shipped qty.".

When a user runs the packing slip report, they will be prompted to enter the packing slip ID number.  How do I insert a conditional expression that says, "Add all of the ship qty. values from packing slips that are less than the user entered packing slip number?"