I once bought a band saw from Amazon with free shipping. (I didn't have Prime, it was just regular free shipping.) I could have bought it locally, but I didn't have a convenient way to get it home.

In my case, free shipping didn't turn out all that great; it was damaged when it got to me. Fortunately I was able to get free replacement parts from the company.

I suspect Amazon probably lost money on that deal since they had to hire specialty movers, but who knows?

"Free" shipping of that sort of thing tends to be factored into the selling price. I'll occasionally find large, heavy items on Amazon that are 20-30% more than in a local store for this reason.