This gobbled up 40 precious minutes of my life, to I'll save you the trouble.
First time I created a discount coupon, it applied just fine, BUT the Total came out negative (probably as Discount - Subtotal). [see the attached screen shots] Freaked me out, since I had no intention of sending people money.
Turns out it is simple: my discount coupon sort order put it BEFORE tax , while by default it was trying to apply the discount to tax as well, thus going nuts.
The moment I set "Recalculate Tax" to "None" everything went to normal. I bet moving Discount Coupon after Tax in the sort order would produce same result too...
Problem solved!
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