question

Ours is a kind of assessment application where any one can taker some kind of online evaluation assessment. So it requires a delivery code to take that assessment. So we would like to sell this delivery code through Amazon. So what we need exactly is from amazon there will our Asessment description public can purchase the delivery code by by the buy now button on the site which will take to our page (ASPX) and select the assessment they need from our page, thus after selecting it it will take them back to amazon for the payment part.. After payment done, we will send them (user) the delivery code they wish.. So how can I accomplish that..