That coupon has been awful temptation, but, I've been restricting myself to getting only missing volumes in previously started series. What has helped, I will admit, is that some of what I want isn't eligible for the coupon.
The primary series I have been buying is Georgette Heyer's detective novels and some of the Morland Dynasty. The Dynasty one is primarily for my Mom and she doesn't want anything past book 18 (thank goodness). I haven't started them yet.