Yeah, but the real secret to PRIME membership is matching a particular manufacturer's brand and loading to a finicky firearm...
Sometimes you come across a firearm that won't like anything you feed it. In may have Failure to Feed, Failure to Fire, Failure to extract, poor accuracy and the rest that goes along with it. So, how do you fix it? By trying a wide variety of brands, loads, grain weights etc.
With PRIME, you can place an order for 15 different boxes/brands of let's say .22LR at PRIME pricing and not be charged any shipping for it.
You will quickly see which sample behaves/functions/shoots accurately in your particular gun. With that information, you can then buy a case (or 3) of that ammo.
This versatility is the true benefit of PRIME...
~Enbloc