Drop ship

We are a wholesaler.  We receive orders from Customer A.    Customer A is now asking if we can Drop ship to their Customer.  Customer B.    How do we set this transaction up in GP so we Can Ship to Customer B but Invoice Customer A?     

For background, we have been a wholesaler using GP for almost 10 years and currently have about 15000 customers for 20m in sales.  Offering drop shipping is a huge endeavor logistically so I'm just trying to get as much information as I can.    Thanks.