Well imagine how it looks from the point of view of anti-discrimination and consumer protection laws: we charge this person an additional $100 because we have some imaginary units telling us they owe us $100... Just trust us. Not sure it will hold in court. If the both sides agree to a specific sum beforehand, no problem. But you can't just charge random amounts post factum without the person having any idea why they suddenly owe those amounts.
P.S. However, if the API includes CoT tokens in the total token count (in API responses), I guess it's OK.