The website and all the part details are amazing. I've made many purchases from McMaster for personal / hobby use so I'm surprised to hear about difficulties purchasing from them.
We are in Ontario, Canada, if it matters. I know Quinn from blondiehacks orders from them without difficulty, so I’m not sure what it is about us that they don’t like. They tend to be very curt/nearing unfriendly in their responses.
There may be difficulties shipping cross-border (customs is hell) - so maybe you "need" to find an address just over the border in the US to ship to instead?
Those also don't usually allow "package" delivery (think UPS) but maybe something can be worked out. Some small post offices will accept a UPS delivery to "unit PO BOX number" but I don't think they're supposed to.