This one often flies a bit under the radar.
If you have any personal or small business US-based American Express card or a World / World Elite Mastercard, you are eligible for a free ShopRunner membership. I would bet 95% of my reader base already has one or the other.
ShopRunner gives you free 2-day shipping and free returns on a wide network of online stores. The store will have an obvious Login to your ShopRunner account” button and once you log in it will seamlessly offer you the free shipping. 140+ stores participate.
You do not have to use the card you have your free ShopRunner account with to pay for your purchase.