As Raffaele and Jeff mentioned in the other thread, you can still create a Free LCCS Developer account and upgrade it to a Paid Developer Account (... unless you run into a bug ).
"Enterprise Customers" are the ones talking directly with Adobe' Sales team and signing big checks (think: lots of 0s) to use the service.
Regarding the bug you ran into, I tested this a couple of days ago on Production and it worked fine (granted I was on a XP/Chrome machine).
We'll try on a X.6 Mac and let you know if we are able to reproduce the issue. Then we will contact the Adobe Store team to ask them to look into fixing this bug.
In the meantime, you might want to try on a different machine to upgrade your account (I wish I could send you a link to add the item to your cart but it doesnt work like that ).
I believe you can also upgrade using this phone number as well: 888-649-2990 (Got it from adobe.com - I didnt call them ... so no guarantee there).
I hope this helps.
Have a great day,
LCCS Quality Engineering
Purchase worked on my 10.7 machine with firefox, good luck with the 10.6 bug. For more information to track it down, I am on firefox 8.01 on my 10.6 machine. Thanks!