I found data hard to come by, but quick googling gives a carbon footprint of around 300 kg for one BTC transaction, and between 25-150 g for one letter sent (in

I found data hard to come by, but quick googling gives a carbon footprint of around 300 kg for one BTC transaction, and between 25-150 g for one letter sent (including production of the paper for letter and envelope). So BTC is around 1000 to 10000 times worse. Would be interesting to see it sourced/researched properly, but at first glance I'd say that, yes, it is much more efficient to send a letter, even if you drive to the mail box.