I think the difficulty is Monzo know where you’ve bought, but not what.
So if I buy from WeAreHeavyCarbonUsers.com and they only sell spoons. And you can easily track that 1 spoon is 4 carbons and they always cost £10, well that’s easy. I spent £40 on spoons. So that’s 16 carbons I need to offset.
But if I spend £50 at Tesco, they haven’t got a clue what I’ve bought, and unless you go through your receipt line by line and someone has an index of each of those items, where they come from, the company behind them etc etc. Then it’s just finger in the air guesswork.