This sounds about right:

"There are a lot of people who have dived into the market who are
desperate to make a margin from it. There are too many snake oil
salesmen in the business."

The thing is though that it’s not just some of the salesmen, it’s the entire idea of buying carbon offsets for your flight.

Virgin had doubts over offsetting products, he said, because its
passengers already contributed to green causes through the £2bn Air
Passenger Duty, proceeds from which, the government says, will be
invested in environmental projects and public transport.

Even that’s not quite right. It doesn’t matter what the revenues from APD are spent upon. As long as you are paying the full price of your actions, as the Stern Review says you should, then you need do nothing else. APD is currently set at the correct Pigouvian level, using Stern’s CO2 numbers, so that’s it. Fly without guilt, you’re already paying what you should be, according to the latest science.