Shipping method unavailable after coupon is applied

Free shipping is provided for all orders on my webstore.

I set up a coupon for 40% off. When this coupon is applied to a $20 item in the cart, the shipping method is removed and the text for shipping method reads “Shipping costs will be calculated once you have provided your address.”

Shipping costs should not need to be calculated since all orders are free.

I noticed that this coupon for 40% off works when the total value of the cart is $35-40 (more than 1 item in the cart).

I can’t create coupons that grant 40% off for single, $20 items. I can create coupons that grant 25% off for $20 items, but not for 40% off.

For instance, go to my store https://www.californiabeardco.com/store and add Beard Oil to your cart. Apply code TAKE40 to the cart to see the shipping error. Increase the Beard Oil count to 2 and the coupon works fine.

You can even add other higher-priced items to the cart, such as the Beard God Pack, and apply the 40% coupon.

But it won’t work for 1 $20 item.

Wordpress v 4.8.5
Woocommerce v 3.1.2

Updating tends to break my store, so we are on somewhat older versions

The page I need help with: [log in to see the link]