Maybe this will help, I just did it yesterday: Once you add the item to your cart, go to the cart, under the price you should see a small box that says "Save 30%" Clip coupon. Do that, then on the payment section put in the code for the extra 20% off.
Thanks. Interestingly, it wouldn't show up in Firefox (my default browser) but did in Microsoft Edge off to the right edge of the item display. The first link on Inkbird's original post only takes me to page of nothing but shopping suggestions based on past purchases.