I just used atmosfair again to compensate my recent flights to Seoul and back. They calculated close to 100€ per single flight at that distance. They invest into projects that actually save CO₂, not just by emission trading (which I cannot understand anyway). They also performed well in audits (Wikipedia), ≈10% is lost to administration,

On the train I met friends who told my that Berlin ministries use atmosfair to offset emission from traveling. Amazing, hadn’t heard of that, must look into it. Amendment: probably not (entirely) true. The Bundesministerium DOES offset its travels, but it uses a different algorithm than atmosfair: heute magazin 2019-09-23.