And now they have added a new fee.....
Now you get a discount if you use a debit card.... Or pay additional to use a credit card. From their website:
Get our lowest price when you pay by debit card.
Prices displayed reflect a debit card discount available to travelers who pay with a debit card. Prices are higher when purchased with a credit card. Debit and credit card prices are displayed throughout the site and wherever the total trip cost is shown.