I had reason to look into this recently. I was taking a long-haul business flight, and wanted to look for constructive ways to mitigate the emissions. "Constructive" in this case means not just blindly paying to plant some trees (I concur with the posters above, and indeed with the range of recent reports pointing out the shortcomings of offsets in general) - I wanted if possible to support emerging technologies for CO2 removal, which have to be scaled up hugely (and reduced in price) if they are to work at the scale we need.

After a bit of looking round and asking advice, I ended up going with a company called Zopeful (https://zopeful.com/blog/carbon-remo...o-announcement) which assembles portfolios of carbon removal credits, with a focus on removal technology.

I hope that's useful for the OP or other posters.

Will my individual action make a difference when [insert adverse "but what about...?" comparison here: China/India/diesel trucks/farms/wasted food/human nature/it's all a hoax/we're doomed/etc]? Not much, no, but funding new technologies will make more of a difference than just another dodgy tree-planting scheme (which has the negative side-effect of making people think they can compensate for their flight emissions for just a euro or two, i.e. that no significant behavioural changes will be needed).

And the "will my individual action make a difference?" question also applies to voting in elections. My individual vote makes no material difference* but at the same time it's important to me that I cast it.

(*at least I live in a country with Proportional Representation voting so my vote is never entirely useless.)