Update: Ok, I emailed them, and they fixed it back to $20.98 without argument -- by lowering the listed price on the Ghostbusters blu-ray to $15.49 instead of $19.99, rather than by putting the coupon back. Still showing only one $5 promo, rather than two like it was earlier. Means I'm really only getting $9.50 off instead of $10, since they factored out the price drop on the CD, but good enough, I guess.
What gets to me is that they removed the one coupon with no notification or explanation at all. If I hadn't actually logged in, looked at my order status, and noticed the change, apparently they would have just billed me at the higher price. Whether stacking the coupons is valid or not, I'm surprised they could do that.
Anyway, you all might want to keep a close watch on your orders up until they ship, to make sure that they don't try to pull it on you as well.