They didn’t say $100/lb, they said a package as small as a pound would cost $100+, that cost would’t scale linearly with weight but it might with box dimensions, similarly to how a single heavier person doesn’t cost more in passenger airlines but multiple people(seats) do.

Also, for an apples-to-apples comparison, try buying that cross-country plane ticket 2 hours before takeoff.

You don’t even have to take their word for it, you can get an instant quote right now from the freight companies (most places that aren’t amazon-scale probably pay about 30-40% of that retail rate)