The only purpose of a coupon is to get you in the store, and hopefully once you're in the store you'll see some other things you like, thus making up for the money the owner is losing on the coupon item. Joanne fabrics run these single-item high-percentage-off coupons constantly.
High-percentage off, single item coupons don't work as well on online stores, it's too easy for people to just buy one item and check out without looking at other stuff.
Combine this with the fact that it requires just as much in labor cost and usually just as much in packing material costs to pack and ship a single item as it does to pack and ship a larger order, and you can see why this type of coupon isn't seen much in online stores.
In an online store, you're more likely to see a coupon with a lower percentage off that can be used on an entire order, or a free shipping offer with a lower threshold.