Add PayPal shopping cart to existing site

We have built a site in PHP which uses a mysql database to displays products, we need to add shopping cart functionality using PayPal as the payment gateway. Products need to be added to a basket within our site before checking out with PayPal, ideally bypassing having to go through the PayPal site and doing the transaction through our site. No CMS required, this needs to be incorporated into what has already been done so needs to be a bespoke cart. Shopping cart needs to have multiple shipping options and discount code option. AJAX would be nice, but not a necessity. Only serious programmers with previous experience please bid.