Through several years of online purchasing (and several pairs of ray-bans), I've done my homework on how to detect knock-offs.

But as this particular order was with Amazon, I had no reason to suspect anything... until I opened the box

I noticed somethings were not right.

They felt flimsier than my other 3183s, which made me notice that the frames were plastic.
Real RB3183s have metal frames.

I also noticed that they were made in China.
Real RB3183s are not made in China.

I also noticed something else, but as this is my last vestige of being sure (and something that could be faked), I'd rather not go into the details and give the forgers the chance to make their fakes harder to detect.

Needless to say, within 20 seconds I'd determined that they were in fact, fakes, and had thrown them back in the packaging to be returned.

Amazon. I expect more from you. You claimed this purchase as coming from you, but the packaging and fact that they were knock-offs has lead me to think they weren't. Whoever is selling these is doing your company a disservice. I understand it is difficult to keep track of all your sellers' products and their authenticity, but if you're going to put YOUR name on it, you're going to be held accountable. Do your research.