Interesting that they refer to “US customary units.” I’m American and have always heard that system called “English units.”
The US Customary system is different than the Imperial system. The old English system has not been used in England since the early 1800s. Yes, you folks may casually refer to the US system as the “English” system, but after losing a few planes and space probes due to measurement errors, you can be sure that relevent authorities are being Very. Precise. about their unit names.
Capital M is used to denote a solution of 1 mole per litre. So obviously they’re selling a solution of this chemical with a concentration of 600 gigamoles per litre.
Sweet! $600 sounds like a great deal for a chunk of neutronium.
Something you’d come to realize if you’re a beer drinker. Bigger pints across the pond!
Learn something new every day I guess. But I will note that the official SI unit for concentration is mol/m[sup]3[/sup] with the symbol “c”, which is why I couldn’t find anything with “M” as the symbol.
Sigma-Aldrich probably won’t sell to you unless you have an account. Something about not wanting laymen buying chemicals. 
VWR - I’ve bought blind, but it’s always been lab supplies, never chemicals. Never tried buying chemicals from them.
Alfa-Aesar - no experience.
I have corporate profiles with SA and VWR. My experience has been that chemical supply companies will not sell to you without a corporate account.
I do buy chemicals from them, but always from a R&D standpoint. Their website sucks - you have to look up a chemical, then call them, then prove you’re a legitimate buyer (read: established chemical company), then prove you have an account. Only then will they let you order.
And SA is never for industrial sized orders, always for R&D. You can get a few hundred grams of nearly everything but nothing on an industrial scale.