Pricing Event -'Enter Shipment'

We want to give discount/accrue on shipment. We have defined a custom event phase based on pricing event "Enter Shipment". System works fine if the entire product line shipped in one delivery. But when part shipment (multiple delivery) occurs system splits the source line and sets the calculate price flag (CPF) to "Partial" as profile ‘OM: Charges for Backorders’ is set 'Yes'. For part shipment, I guess system first splits the sales order line (when we run Interface trip stop (ITS)) and updates all the sales order line CPF to ‘Partial’ related to source line where from it splits and