Carbon offsetting becomes effective when it causes less carbon to be emitted. It has to be so expensive that it makes the consumer of the carbon to think twice. This can only happen if the offsetting is mandatory. The further up the chain it is the more effective it is as well.

We might be able to death by a thousand cuts the CO2 problem. The more sectors we inconvenience with "green washing" type extra costs, the more attractive greener alternatives become, and the more funding potential solutions get.

The article suggests investing in a strategic fund instead, but it's really the same thing and has the same problem as the CO2 offsetting which is that the calculation of the amount of offset CO2 is not realistically calculated. The charities might have unrealistic long term CO2 offset estimates, but making the amount dependent on how much your budget allows for doesn't really make things better.

If you fly economy from LAX->AMS and back, you emit ~2500kg of CO2. Easiest way to capture that CO2 is by planting trees. Say you want to capture it within 5 years. That's about 50kg per tree, and about a third dies so you'll need to plant 150 trees to cover your flight, which is $150 on teamtrees.org. Provided of course they actually plant it within the next year (do they?).

That's already $50 more than that GCC site suggested you spend if you've got plenty money. Trees grow somewhat superlinearly so if you give your tree 10 years you need maybe $50 worth of trees.

Anyway, it still won't save the planet, that only happens when all of this is mandatory and not just for flights but for everything.