Ecommerce/Shopping Cart API

Looking to have custom API script written for retail ecommerce site / platform. API would need to consist of customer purchasing virtual or digital products online. System would need to pull digital product via api from source and both: 1) email customer e-product after payment 2) store copy of transaction on customer profile - which i believe most shopping carts already do. I am currently working with a shopping cart that has very simple ability to create mirroring sites & also integrate particular products or pages onto any other existing site via a simple script. I need to stay with a cart that has this ability. I'm open to using a cart or ecommerce that a programmer is familiar with provided it meets these qualifications amongst the normal needs in an ecommerce site. I have the API docs for one source. This project will likely need a second and third api connection which I will look to preferably get added by the same programmer on this first api. My intention is to start hire of 2nd & 3rd api's within 90 days of the first one being complete. * Timeline of how long this project will take is important. The quicker the better.