Sad, but true. I t plays remarkably little role. Actually, usability is much more important on websites than with physical products. On a web, if the site is unusable, you just leave. It is common knowledge that shopping carts are often abandoned by web users.
Usability is always secondary.
It's never the most important thing about an experience. I will accept poor usability if I get what I need, if the total experience is great. I will reject perfect usability if I am not rewarded with a useful, engaging experience. Most sites make it easy to fill a cart, but difficult to complete the purchase. One has to fill in all sorts of information that has nothing to do with the purchase.