Modify existing Cubecart

This project is to modify existing Cubecart (version 4) store. Actual Cubecart: When order is shipped, a mail is sent to the customer to confirm shipment.
(there is a shipping label you can click on which asks if you want to print)

Modification: When order is shipped, an invoice is generated and saved in PDF format, then uploaded on the server. A confirmation mail is then sent to the customer, including a link to download this invoice. At the same time I can save the invoice to my computer.