Most people who are volume licensed get a login to Microsoft website, so you don't need the media you just download the ISO.
And OEMs generally don't provide media any more, but they can and a lot do, provide an "image" of the windows installation. On DVD and some automated mechanism to restore the machine. so the user doesn't have to do
<FavouriteImageProgram> tack tack restore my os
Which imo is better for the end user, because they don't have to actually sit through a windoze install and then find loads of drivers etc. and because the image and process is built for that machine its going to work, unless of course theres a hardware failure.
Some el cheapo metal badge maker, who throws a £10 case/PSU in with a £15 motherboard probably doesn't do this, but i think in this situation you have more to worry about than no windoze cd.
If you want to resell boxed product or Volume licensing to them, you would probably be better going through a reseller yourself and tacking a nominal fee for administration. I doubt they will deal directly with you for one customer. anyway with out the quantity of business you are not going to be competitive.