Here is the first relevant screen of the Baen checkout. Your Bundle is in your cart. You need to put your Coupon Code in the text entry field at the bottom that I have masked in bright blue. See attached screen-shot
PRIOR to entering the Code and clicking "Update Cart" the Subtotal was $18.00. After entering the Code and clicking "Update Cart" the Subtotal Including Discounts is $0.00.
IF your Coupon Code is invalid this
is the stage when those "Red Letters of Rejection" will appear, and your subtotal will remain $18.00.
ONWARD to the next screen!