The cost is a factor, but the 7-10 day delay is far worse for me and it’s ~$10 shipping to get 2-3 day delivery.

Also, do we know for sure the Ingram warehouse employees are better treated than Amazon?

To be honest, Amazon shipping works because it's a monopoly. They don't care how much you buy, it's almost the same for the company, they have to hire the same amount of staff, and same paid hours, at least in urban area. But for any independent store, they have to negotiate a rate with a shipper, which would charge a flat base rate, and overweight if there are any. So it can be difficult for them to offer quality and fast shipping in terms of cost. In a way, they are less efficient than Amazon.

Bookshop fulfillment is handled by Ingram Content Group, a massive book distribution and logistics company, and itself the biggest distributor to independent bookstores.

Disclosure: I work for a child company of Ingram

The reasons makes no difference to an end consumer. They just care about the services provided.

I don't disagree in theory, but I don't understand how Bookshop protects indies from Amazon in a way that selling their books directly doesn't, especially if they don't offer an advantage on price or convenience.