There are lots of suggestions here of how it's done but it's essentially really really simple.

There are 2 pieces of information that need to be joined.Google have your cookie and email and MasterCard have your address and probably email. If both sides have your email then job done. If not then they can use your physical address via a data broker. All it needs is some e-commerce sites that allow cookie syncing and have a privacy policy that allow them to sell that part of your data.