If they ARE selling to a big box, you can get all their orders through EDI (import them as SO's, send them back out as invoices and shipping notices) and they actually will be separate SO's with the distribution center (or final destination store) as their ship to address. But you don't care that it's a hundred SO's because everything is automated. HighJump (True Commerce) brings in the orders, you fill them, it sends the shipping info and invoices back to the bill to party. Don't be surprised if they aren't already doing some of this EDI on some website but don't realize that's what it is. With True Commerce, you buy a 'partner, like Walgreens. And however Walgreens does their orders and wants their invoices, True Commerce already knows their way and uses their way and it doesn't even matter if Walgreens uses X EDI provider. Everything can go through your VAN (network) and come out looking the same. (Mapadoc and SPS also have EDI integration for Sage 100)
Sales Tax? I don't feel like EDI would address that, but my clients sell to resellers more than direct to consumer.