Some places use credit card info to verify age. If something is different on your card than on the info you entered during checkout, their age verification service might return that message. That's a different service than their CC processor, so it wouldn't be rejecting your card, just unable to verify that it belonged to you, which is done not to combat fraud, but solely to verify age. Leaving out a middle initial could be enough to do it. Make sure your name is exactly like it appears on the card and that your billing address is exactly the same as you entered on the site.
A while back, Madvapes went through a period where people were complaining about problems with their cc info being used to ring up fraudulent charges. None of it, AFAIK, was traced back to Madvapes, but I can see them putting some new service in place to tighten up security so that they don't get suspected of leaking CC info.