Looks like the CO2 emissions of sending a letter are 25 grams or so. Meanwhile, an email with a 1 MB attachment is estimated to emit 19 grams of CO2 [1]. I think the real numbers might be off by a factor of 3 or more - there are so many variables: opening the email on a desktop PC and watching a video and sending it to friends would use much more, and maybe you mail the invitation to someone who doesn't get much mail and lives out of the way so the mailman has to drive 5 more minutes just for one letter.

In any case, the CO2 emissions of the invite are negligible compared to other things: say on the night of sending the invitations you have a lamb dinner with your fiance and you each eat 12 oz of lamb, that dinner was responsible for 27 kg of CO2 emissions vs. 120 invitations would be only 3.5 kg.

In turn, that would be far less emissions than feeding everyone 12 oz of lamb (1607 kg of CO2), which would be far less emissions than that produced by the guests arriving (say 7% take transatlantic and 28% take cross-country flights, all economy) - a whopping 73920 kg CO2 [2].

[1] https://90by2030.wordpress.com/2015/12/09/carbon-footprint-l...

[2] BER-NYC roundtrip is 2.4 tons/passenger in economy = 0.07120=20.16 tons + SFO-NYC roundtrip 1.6 tons/passenger in economy = 0.28120*1.6=53.76 tons. Total 73.92 tons.