The Venmo API is made up of two different public APIs. "The Payment Links API provides a simple method for sending invoices or suggested payments via any medium (Email, SMS, Facebook, Twitter). Payment links have 3 optional parameters and are constructed by adding these parameters to the base URL. The Receipts API allows every Venmo user to send receipt data for payments they receive to a third party webservice. By default, users get receipts via Email and SMS, but the Receipts API provides a simple method for automating followup actions to payments (like collecting a postal address for fulfillment or automating digital fulfillment via email)."
Please note, from the company: "The Venmo Developer API is, and will be, available to all existing users and we will continue to support those partners, including Braintree merchants who use the Payouts API. We have, however, discontinued accepting new beta users of the API as we are focusing efforts to open up the beta to pay with Venmo. Focusing our efforts on allowing users to pay with Venmo within select merchant apps allows us to provide many of the capabilities previously powered by the API, now with optimal user and developer experiences alike. "