I don't think this could be accurately described as merely a _vulnerability_ disclosure. These web stores are already compromised, therefore anyone that makes a

I don't think this could be accurately described as merely a _vulnerability_ disclosure. These web stores are already compromised, therefore anyone that makes a purchase is exposing their payment information. Furthermore there is no "responsible" way to contact thousands of web stores across the globe. The best case for disclosure is Google's Safe Browsing beginning to warn users immediately.