Hi Otis -
It's not that you "can't" use the coupon on sale items -it's just that it's not
usually worth it. However, if a sale item were 20% off and you had a coupon for 40% off, they would certainly honor it. You just can't take 40% off the sale price.
I bought my extra large cutting mat at 40% off which saved quite a bit.
-Judy