Add Event Date to checkout_shipping.php

[Guest]

I am trying to modify the checkout_shipping.php page to include a field for an event date as my business works in rentals rather than purchased items. I am familiar with PHP and MySQL but I am use to form post and get data and am not familiar with how OSCommerce transfers the data around. How do I grab the information from the checkout_shipping.php page and transfer it around to do date/inventory checks and final checkout to store the Event Date in the order invoice?

 

Any help will be much appreciated.