I'm currently working on a tool that will be used to add some parts to the checkout page.
I have been looking at the core files but so far I haven't found anything about how to actually pre-set fields to the checkout (shipping details section).
I have no problem adding products, etc.
EDIT: This script is beeing used for a customer to reorder the same products again. By code I can add the products to the cart. This question is regarding the customer details such as firstname, lastname, address, phone, email and have them added to the cart.