Using a website like this to generate valid but fake credit card numbers is specifically for testing e-commerce applications and should not be used here. That would be immoral. Source: almost 2 years ago

Https://cardguru.io this website does a great job of explaining the algos and everything else. So you dont have to call your bank or do any of that nonsense. You can just literally make everything up when signing up for a free trial and not even worry about being charged. Source: over 2 years ago

They may be using something like Sift for security checking and something of yours was flagged. Source: over 1 year ago

But sorry to break it to you, this has been done at a really large scale already although most consumers are not aware. One big player here is https://sift.com/ Almost every major retailer uses their service exactly for the reasons you mention. Source: almost 3 years ago

Reddit, for one. A pretty big list on their homepage. Source: almost 3 years ago