What’s the most environmentally-friendly way to ship a package across the country?

I’ve got several packages that I’m planning to ship from the Midwest (US) to the West Coast (US). This has me wondering what is the most environmental way to get them there. Ignoring costs for the moment, what will have the lowest impact in terms of pollution and carbon dioxide.

Is it more environmentally friendly (more green, lower carbon footprint, whatever) to ship it via ground transport or via next day air?

How about overseas? Is it more environmentally friendly to ship via, well, ship or airplane?

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