Maybe it is different in the US, but in the UK the charges being inconsistent is the reason I do not use One Click. (Amazon UK not being as good seems to be a theme in these comments.)
I don't think that's the case for Amazon Prime (UK customer)
In my experience the price shown and delivery time estimate is always accurate, and as a Prime customer the delivery fee is included in your subscription
In my experience the price shown and delivery time estimate is always accurate, and as a Prime customer the delivery fee is included in your subscription