They were forced to collect sales tax, it's not as if they just decided to fuck their customers one day.

Every prime item I order comes in two days, period. Do you live somewhere rural?

I'm seeing plenty of diapers with Prime shipping, for example http://www.amazon.com/gp/aw/d/B00DFFT6IO

You realize that the "add-on" thing is actually a perk? As in, you'd be paying regular shipping for that item otherwise.

Can't speak to the returns issue.