Urgent and simple: Modifying Zen Cart

Hi, We need an online store for our software products. We have a number of software packages, and each of them are license protected. we have a MySQL which holds the license activation code and download path of the products (note that there is a separate file for each buyer with its own unique activation code). The online store must be give us the ability to add and remove products. The payment must be through PayPal. Once the payment is confirmed, an email must be sent to the client containing the download path and license code of the first unsold file of the client's purchased product. Please explain your solution in your bid.