Custom feature for cart page

I have a site that i have a comparison site i run, built on xhtml and php only, i help some customers shop as well , where they add the comparison product in a basket and when they submit it i do the shopping, but now i would like them to pay a percentage before they submit the order. i thought it might be straight forward, but from this post i have read, whats your advice.
This is the link to the shopping list page
http://reacheasy.co.uk/youraccount.php.

Thanks