How to add an editable field for the current product price?

[samz]

Hello,

A non programer friend want a philanthropic (non-profit) PrestaShop website, that I'm installing/configuring for him. He would like something really weird like this:

on the product detail page, the price should appear in a editable field, and the visitor could be able to change the price typing a new price (even zero) in this field. Then this price (in the field) is used in the checkout process and the final invoice. The default price in this field is the price of the default combination. Clicking a combination will update the field value with the price of the clicked combination.

1. Where (in which PS files) should I start to add this crazy option?
2. Should I do this job in PHP or JS or both?
3. Where/when are stored the product id, quantity, combinations while the user is shopping? I guess in PHP cookies.

The first draft using PrestaShop 1.5.6.2: http://www.airfirst.ch/s/

Thanks and regards.

Edited March 5, 2014 by samz (see edit history)