TIL Venmo uses GraphQL.
It's interesting to note, too, that the current Venmo website posts to https://account.venmo.com/api/eligibility to get a token and then separately to https://account.venmo.com/api/payments to perform the actual payment. Those endpoints and shapes are different than what's in the script, which posts to https://api.venmo.com/v1/payments (https://github.com/Integuru-AI/Venmo-Unofficial-API/blob/a28...). I wonder if the v1 API is an older one used for some other service (the mobile app, maybe?).
Thanks for sharing, OP.