Inbound EDI 850 - How to handle when our CSR needs to determine Branch / Plants

We have a customer that wants us to implement EDI with them.  For us, this would include an inbound 850, outbound 855, inbound 860, outbound 856 and outbound 810. 

We are struggling with the initial piece of the inbound 850 because when this customer provides the order details to us, they don't know what type of sales order it is (SA or SO) and they don't know at which Branch/Plant we will be producing the material.  It's not as easy as saying that a certain material always comes from one branch/plant because our customer service currently makes the decision based on what is going on at the various plants and which ones have the bandwidth to do the production.