I avoid JoAnne's because I hate their coupons. There is always some problem with them -- good for only one item, the coupon has expired, or as in your case cashier misapplication of the coupon. I realized I was spending *more* money, not less, when I went to JoAnne's with coupons so I just stopped going.
Now I purchase rotary blades in quantity off eBay for $1.25 a blade or so, and I have to make the purchase only once every year or two. We have "Amazon prime", so I often buy quilting books from Amazon. Even if not quite 40% off, they are only a dollar or two more that way.
Rulers -- I have such a collection already, I don't need to be tempted any more!