For any ecommerce store capturing the credit card number is the most important step. Therefore, it makes sense to have the most user friendly credit card capture interface. And Google has nailed this.
When you first click inside their credit card box they show payment icons in background of text box. Like this–
Then when you type the first digit of credit card they recognize the card type. For example, if your first digit is 4 you have a VISA–
If your first digit is 5 you have a MasterCard–
Also, as your type your credit card number they add an extra space after every 4 digits to make it more readable (it also matches the way the numbers look on your physical card)–
What’s really crazy is that they seem to have some sort of realtime card validation going on because entering the fake 5555 5555 5555 5555 number threw up the red border error message. I have no idea how that’s done–
Anyway, the point is that this degree of attention to detail is what separates the campions from the rest.