Add "request invoice" in checkout!

I have an online store based on PrestaShop 1.5.4.1; in the Checkout process, in the Address Section of the "Quick Checkout Page" a checkbox saying "Request Invoice" have been added; the checkbox appears only when cart total is >= a configured amount. A user could request an invoice ONLY if: 1. He has a Fiscal Code or Vat Number on one of his Addresses 2. Cart total is >= a configured amount The project is about making the checkout process and invoice generation to work correctly following the above conditions. I will choose cheapest!