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Sometimes, when I encounter a problem making a purchase from the Renderosity website, I find that there are issues if I'm using Firefox as my browser.  I just encountered this when purchasing Blacksmith with the July 4th discount coupon.

The solution for me was to log out and re-log back in using MS Internet Explorer. 

Then things worked just fine and the discount was applied.

Firefox also has problems on Renderosity's website when you're trying to use Paypal.

Again, the workaround is to use IE.

It would be nice if Renderosity and Mozilla would work together to resolve this.