Theoretically speaking, payment could take the form of data as part of an enterprise agreement on rates charged. Notably, the OpenAI API privacy policy specifically states...

> We may also aggregate or de-identify Personal Data so that it no longer identifies you and use this information for the purposes described above, such as to analyze the way our Services are being used, to improve and add features to them, and to conduct research. We will maintain and use de-identified information in de-identified form and not attempt to reidentify the information, unless required by law.

The fact that Mixpanel has this data in non-de-identified form is suspect to me. Granted, my entire comment was clearly tongue-in-cheek. Although I think it's possible that OpenAI is selling this data to get a discount on Mixpanel usage, in reality I understand that the more likely explanation is that whoever was responsible for managing this data is completely and totally incompetent.