You need a way to remove a discount code that may have been incorrectly inserted.  For example: I want to offer a discount for not using PayPal and using another option where I don't incur fees.  The code is easily created and applied, but if they change the payment method back to PayPal, there needs to be a way to cancel the discount.  I found that you can negate a discount by putting a blank space in the discount code box, but you need some instruction for the user to know how to do it.  

Maybe some text like:  To remove a discount enter a blank space and then the "Apply" button
or better yet: Create a "Cancel" option next to the discount box

or even better allow this to be option in setting up payment methods and applied automatically when a non PayPal payment method is selected and cancelled if PayPal is reverted back to by the user.

Just a thought.  Thanks