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Credit Cards Are Vulnerable To Brute Force Kind Attacks
(metin.nextc.org)
This is a most excellent place for technology news and articles.
You might want to try something like Anubis on both the signup and order pages. Real users will either not be stopped, or will only hit it once, and no user interaction is required to continue, but bot users will be slowed down enough to, hopefully, disuade them from returning.