Magical Express drivers are NOT Disney CMs - the service is provided by an outside contractor which provides the buses and the drivers. So, yes, ANY time somebody handles your (well, my) bag or bags, they get tipped.
We gave a tip last time, but we got our bags ourselves at baggage claim so driver had to stow our bags and stroller under the bus. We gave him a dollar per item when we got to resort. Your bags may not be on your bus with you if you are having the "traditional" ME service, so I don't know what to do with that situation. I'm sure someone else will have the answer.
I don't really know the answer to this, but I would say $1 a bag. I saw your post before, but thought that you were asking about tipping the Disney drivers in general and thought, "Who does that?" I had to look at your question again....
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