I alreafy built this for a company I used to work with. A sheet contains the invoice design, you enter orders there, client info and product info are pulled from 2 separate sheets. once finished, the invoice can be printed, sold units are removed from inventory, sold units are listed under the sales person (for tracking of who sells what), accounts receivable entry is generated, collections calender receives new order payment plan details, sold units are registered under the client (to see who buys what), and finally, workbook saves automatically. All this with the click of the "print invoice" button.
I can show you what this workbook looks like, show you a demo and you tell me what you like and dislike of it to modify it to your needs. 3 days should be enough to modify, deliver, field test, and make corrections or improvements you may want.