You just send him a message on that page and he (or his wife) will reply. Payment and such are done 'officially' and you get the paperwork. Not that you should have anything to fear, I know the guy and his wife personally and they are great people.
May wanna check with your group though. His backpacks are the 1801 model, and the 21e carries the 1812 model (Remember, the one that was
never made
). Some groups prefer uniformity to accuracy. Also check that you get all the strings, like the two capote-straps (Unless you already got those) and then there's a big strap that goes all-over the middle, though I've seen people without it. Probably because it only has real use keeping your dinner plate on your back (Still, looks very campaignish)