all in all it's gotta be cheaper to ship time-insensitive things via a vehicle that consumes no fuel. I'm curious if they can turn around and monetize their lack of emissions thru some kind of carbon markets or whatnot
Unfortunately, these ships only burn the nasty fuel in international waters so the process of getting them to pay for emissions would have to go through international maritime law, specifically MARPOL [1]. If you want an idea of how difficult that would be, check out the implementation and enforcement section of that page.
Labor cost per ton will be huge.