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!