I buy things from them even if it's double price, since I know there's no drama with their orders. It just works. Everytime.
They are the defacto standard that I have for a great e-commerce experience. When I need design /UI tips for my website. McMaster is always referenced. Never Amazon...
And my favorite feature: Nearly every part comes with complete data sheet and CAD models on the product page.
One anecdote: I ordered a large mounted bearing for an electric vehicle drivetrain, about a $250 part. Upon opening it, I was confused because the bearing didn't seem like it was seated correctly -- It was slightly off-axis. I called McMaster and they sent a free replacement the same day, no questions asked. I later realized the bearing was self-aligning and needed to be greased/mounted to align properly, so it was 100% my mistake and the first part was completely fine!
Another distributor that is super strong on the fulfillment side is Digikey. I had a tour of their operation several years ago and it was amazing. I saw orders picked and moving to the UPS truck 10 minutes after payment. Web site not nearly as good as McMaster though.
As a young kid I remember spending hours flipping through the catalog when visiting my dad at work, marveling at the then hand-drawn illustrations of each product, wondering how a place could somehow sell what seemed like everything.
Even today, I use their site in a similar way. If I need to build something, fix something, or figure out what kind of parts for a project even exist, their site is usually my first stop, sometimes followed up with a text to my dad to ask him how he might solve it—which sometimes prompts him to walk out to his garage, pick up his now-dusty old copy of the printed McMaster book, and flip through it to find the right part.
They get things to people so fast because they have trucks they send directly from their warehouse to the local UPS distro facility to you every day. Amazon does that today but McMaster-Carr has done it for years.
their warehouse hires people with college degrees, PHD's to pick and pack, they have amazing perks, and a really nice workspace.
Someone needs to start an international shipping startup that forward ships McMaster stuff, or they need to open an international shipping team.
Dunno why they were so harsh. Perhaps terrified of accidental sanctions regulation issues. (Even though I have nothing to do with North Korea, Iran, etc)