Need advice in choosing between making a Kit/Package or an item group

We sell an item that has a core which gets returned. When the item is sold we also charge the customer a core charge which is then credited/refunded back when the old core is returned. The item itself is always drop shipped from our vendor to our customer. and the core is returned directly to the vendor and the vendor notifies us that the core was returned by issuing us a credit. I need to figure out the best way to implement the item and achieve the following;

a) Set the item item to automatically produce a PO from the sales order for a drop ship