Fulfilled by Amazon is not the same thing as Sold by Amazon. The former (and the problem area) are the third party vendors in the Amazon market. All I can say is "caveat emptor." To myself, even.
I routinely check the marketplace vendors for pricing. Fulfilled by Amazon is nice because I still get the product quickly without having to pay sales tax.
So this is a good heads-up for ANY use of the Amazon marketplace. Thanks!