indiglo is right that you can't stack coupon codes with discounts, but it looks like those leggings aren't on sale right now. If you become an advanced reviewer you can do a buyout that will let you get them for 30% off in exchange for a review, or you can look around and find plenty of people's affiliate codes that will get you 15% off an item that isn't on sale. You can even get your own affiliate code and use that if you wanted.
If it's on sale you can't use a coupon with it. I wondered the same thing a while back and tried it, but it doesn't work like that. But luckily Eden has some very nice sales! And these are currently on sale now.