When you are on the final order review page, before submitting your order, click on edit shipping address and then hit Update shipping address. It should now list your shipping fees as FREE.
Also, I found that if you checkout as a guest using the same email and then entering your WC rewards #, you have no problems using your $20 rewards. I was having problems with using them after placing one order where I used a $20 reward. Basically, they worked one time and after that I wasn't able to use another on my regular account. They are not tied to your account either.
Yep. Just received an order for a sweatshirt and socks that was under $20. Also just had another order for jordan compression shorts at 19.99 shipped. I wasn't charged for shipping on either order.