This is true about almost all human groups/movements though, and I'm not sure that environmentalists are more guilty than average.
But people who drive to work everyday still try and lecture me over nothings like putting a laptop on standby overnight.
I live within 8 miles of the office and subsequently put about 4,000 miles on my vehicle annually. I consume less fuel than 90% of the people in the office with new 'environmentally friendly' cars who live 40 miles away, yet it is I who receives the rash of shit. /rant
Micro-optimizations are often easier to wrap our heads around, so we tend to want to focus on them. They're also often situations where you can get a clear win rather than a much more imperfect improvement. In development, it's sometimes called bikeshedding. You see it often in financial planning where people would rather cut out their morning latte than focus on investing better.
This behavior makes little sense, but it does appear to be a fundamental human tendency and those who are better able to maintain perspective definitely have a huge advantage in life.
Dropping paper invitations has very little real inconvenience factor, so people do it and tout their environmentalism to receive their karma from their friends that care about that type of signaling.
When it comes to making a real sacrifice (e.g. not inviting people who would fly to the wedding), you will find very few people willing to do that.
It's the reason the prius is so incredibly popular compared to fully electric cars. It comes with the environmentalism bragging rights while still burning plenty of fuel so there is no change in lifestyle. Toyota was even nice enough to build it in a weird shape so people can easily see how forward thinking and smart you are.