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Did it say it was declined, or just an error page after trying to purchase?
I had something like that happen last night. This is a my first purchase here, chose the FICO Standard $15.95, but found a discount code on the internet too that worked.
I'd enter my Visa Debit card, hit submit, and it took several minutes before I got an error page, stating sorry about this, that there could even be a temp hold on your card due to the error, etc. But nothing about the card itself being decline. Just looked a like a simple timed out error with a bunch of cautionary words.
I tried it again with the same card to be sure, it did the same thing. So I gave up (about 2am California). I wonder if the coupon could have caused a problem. And I chose Transunion, not Equifax.
EDIT: Finally worked fine a hour or so ago.