How to create another page after checkout to add extra data to product table?

Hi,

I'm working a project which requires a customer title, message and url to be stored after the product has been purchased. I believe the best place to put this functionality would be on the Thankyou page or as a link to the submit page from the Thankyou page.

Before I go ahead and try this I was wondering (being a complete zen cart noob) could give me some tips on how to approach it?

I've read through the dev docs and relevant tutorials but I'm still a bit lost. I've found the tpl_checkout_success_default.php so I imagine I would copy this to a custom theme and modify that right?

The new functionality will only be available for one category so I will need to check what the product category id is and what the product id is so I can create the correct sql query.

I'm also concerned about security and would like the functionality to use data stored in the session and not pass any pages any parameters via post or get.

I basically need a general process to follow and I can probably work out the rest. Any help would be greatly appreciated.

Thanks,

eb_dev

Zen Cart 1.3.8a
Patch: 1::
Database Patch Level: 1.3.8