Most credit card companies require the four digit year.
To change it to 2 digit you would probably just have to add "20" in front so that it always passes it as four digit. The requirement isn't eCart's it is the payment processor, so you'll have to pass 4 digits to them but you can get it from your customer as 2 digits and then add the front two digits.
That will work at least for the next 60 year or so until people start having cards that expire past 2100.