If you're selling a B2C service (so the really horrible tax rules apply) for say £10/month, that extra £0.20 is another 2%. Stripe's total cut for an international sale with currency conversion is then nearly 8%, even if you don't use any of their other services with their own extra costs and you never have any refund or chargeback costs to amortize.

Stripe is crazy expensive these days.

(There are other parts in the fees charged by other services as well, but those also aren't like for like comparisons. I'm just saying that 5% as a baseline wouldn't necessarily be that high compared to services like Stripe.)