When I go to a pharmacy, I trust that they will make the medication that matches my prescription.
Amazon is _notorious_ for having counterfeit items commingled with the same bin code (I might be using the wrong term), e.g. for cellular phone chargers, and even things like t-shirts.
Because of this, I share the GP's concern that Amazon would to a terrible, un-trustable job at preventing such commingling of pharmaceutical products. Sure, their pharmacy suppliers might make them right, but that doesn't prevent some random jerk (or just someone w/ crappy safety practices?) from posting a counterfeit with the same labeling.