I just wanted to update my experience, since my original post quoted mis-information from Amazon.
I've started my Christmas shopping, and decided to try the No Rush Shipping concept one more time. I bought two more items, and anyone keeping score would know that I already had two (belated) shipping credits from the process. Yesterday I bought a Kindle book for $11.48, but was charged $7.48 -- proof that the multi-discount benefit still works.
And proof that Amazon isn't keeping their customer service reps informed about the details, either.
Sorry for any confusion that I caused.