[mattmaroon]

Saving one click can increase conversions, and if you're doing any real volume even a tiny fraction of a percent can more than pay for the time involved in implementing something like that.

As to why it can't be detected and just neither asked nor shown, it's handy to show people where the CVV2 number is, and how many digits, as many people don't understand what it is even after repeated use. You'd be surprised how often I call in a food order with my Amex and have them tell me it's a 3 digit number on the back of the card. Even someone who takes CC numbers all day doesn't know that an Amex's CVV2 is a 4 digit number on the front, so you can't expect average Joe Amex user to know better.

Also it's nice to group the digits properly when I'm typing them in. So it's nice to have a form at least detect it, even for me.

[adnrw]

Funnily enough, the form in the original post validated my credit card number as Amex and then validated a 3-digit CVV number as OK.

[davekt]

Some amex cards have a 3 digit cvv [1].

[1] http://ecommerce.shopify.com/c/shopify-discussion/t/heads-up...