I forgot to mention when I bought my upgrade the other day the same thing happened to me. It would not add to cart, but I did get it to get in my cart. I think I hit refresh on my browser and it worked. My reveal 8 is working great. But I did experience the same add to cart problem.
I just tried it again and the same thing happened. What worked for me using IE 8 is to right click on the add to cart and click open in new tab or window (either one works). Now I remember that's what I did to get it to work. Are you using internet explorer 8?