Well, I just checked, and everything appears to be ok on our side. When you say "they're redirecting to Eventide", what do you mean exactly? When you first open the store? When you sign in? Or when you hit the Confirm button to confirm your purchase?
We had a few other users with the same problem last week. See the forum thread: http://forum.eventide.com/cs/forums/t/9408.aspx
One of them also purchased about five algorithms and then they couldn't purchase anymore. When we contacted Amazon about it, they said that in one case the credit card company was simply declining to process the transactions. So, it seemed like the algorithm purchases set off their fraud detection triggers. You might want to give your credit card company a call and see if that's the case. We can write Amazon for you about this as well, but they won't process the email until Monday, and they might possibly just say what I've just told you.