It would be wonderful if it was actually possible to use a coupon after you enter your email, as the coupon field appears below the email field. However, once, the payment dropdown activates, the coupon field is locked and a user can not enter the code.
For a bug report: I have only one coupon code good for "free" and no other coupon codes active. When the user enters his email, they can not enter a coupon without reloading the page.