B2B Portal - Allowing Customers to Upload Their scanned PO (Attachments)

Many customers (retailers) generate a "Purchase Order" in PDF or JPEG. When we deliver the product they want a copy of our 1) invoice and a 2) copy of their Purchase Order at the receiving bay. 

Requirement Request: 

At the time a customer is placing an order can we enable the customer to upload a PO or Document?  Then have this show up in the transaction "Attachments" section on our side? 

This would be critical in allowing us to use Dear.