That seems like a pretty fickle data source to build a product around.
I have multiple credit cards. I only use one of them for online purchases. It happens to be my oldest card, the one I got when I was 18 years old. Therefore it qualifies as an "introductory" card available with little/no credit history. Would you assume anyone paying with their oldest card has a low credit score?
There are so many factors that go into a credit score that it seems asinine to base an estimate of it on the specific card used for a particular purchase.
Sure you can get the "tier" of a card and know that the issuer only grants that card to someone meeting a minimum credit score. But you know nothing else. Some people have a 750 credit score but have never applied for a second card. You don't know how the age of the credit line, its utilization, its size, its payment history...
This "reverse credit score" estimate seems like complete snake oil. If credit checks are actually an important part of your business, factor them into your costs and just pay the $50.