In your case, you will get orders from SAP system and will send invoices to SAP system. If your customer has a SAP BC installed, you can exchange between SAP system and your system by XML, or they have an EDI system, the two system can communicate through EDI interface, at system level sap use IDoc for data communication. I don't know what kind of interface your customer offer to you, so it is difficult to say how you can integrate with their system. If your customer open their SAP system to you, perhaps you will have to develop a middleware with RFC support for realtime data exchange between your two systems, for SAP side it is easy, sap can generate a C language interface for you, then you will have to develop the part communicate to your side.
You can reach me by qishilau@hotmail.com.
You can reach me by qishilau@hotmail.com.