Not to rain on the parade here, just a cautionary tale that is intended to be helpful...
I would say take a look at Cool Effect which launched in early 2016. Charity at a glance, but it's also a bottomless-pocketed family from Marin. It cuts itself checks from the Overlook Intl Foundation, run by the same people. Both orgs have same CEO. All fine...
But look at the 990s... They have thrown the kitchen sink at this and have probably dumped $10+ million by now. $800K+ in PR and marketing before they even launched.
Yet they've only sold well under 1 million tonnes over 3 years. In other words, back of the envelope they've lost $9 per tonne, even when being a charitable donation.
Also, divide their 838,715 cumulative tonnes noted on their home page by their 533,115 cumulative members. 1.57 tonnes/member over 3 years, or a half tonne per member per year. Not anywhere close to the average US footprint.
Half the country doesn't believe in climate change, or it's not a priority, or is living paycheck to paycheck, or all of the above. Offsetting is not anywhere on the horizon of their hierarchy of needs. Offsetting is complex, misunderstood, and controversial (see our op-ed's n TheHill.com for a taste).
When you finally get beyond those nesting subsets, there's enough market size to go around in theory but it's extremely competitive.
