The screenshot shows how Tweets with purchase options will display a ‘Twitter Commerce’ label on them on the news feed.
Users would then click a ‘Buy with’ button that would prompt them to enter their credit card details.
The mockup also shows a package tracking feature that users could follow on the map and receive updates about.
Re/code discovered the documents on Fancy.com on an area of the website that was not password protected.
Fancy.com is an online commerce site in which Twitter chairman and co-founder, Jack Dorsey, is a board member.
According to Re/code, Twitter Commerce would be powered by Stripe, the online payments company which recently closed an $80 million funding round.
Neither Twitter nor Fancy.com has commented on the leak.
Twitter will report its first ever quarterly earnings next Wednesday with investors and analysts sure to be interested in Twitter Commerce and other plans for bringing in revenue.