Fascinating -- and a little troubling.
  1. I changed ShedKitStore ot_coupon as suggested, and that fixed it!
  2. Further, I changed HardwareDog ot_coupon settings, which originally had Include Shipping set to false. After I changed it to true, I finally reproduced the problem in another Zen-Cart.

I guess this means ot_coupon is a bit vulnerable in that customers who do a post-redemption cart revision can apply coupons toward shipping denied products, contrary to store owners' intent.

THANK YOU, Linda, for greatly accelerating the solution to my problem. In the process, you have also discovered information that may help others prevent shipping mishaps due to an obscure Achilles' heel.
Quote Originally Posted by Ajeh View Post
Check your current settings before trying this ...

What happens if on the Modules ... Order Totals ... Discount Coupons ot_coupon ... if you edit that and set everything to: