Add a shopping cart to my websites

I need to add a shopping cart to a number of sites i run. As it needs to work across a number of sites all running off the same cms ( its a custom CMS) I am thinking that a shopping cart that works on a pop up window would be a good solution. I would like bids to use an existing solution or to code a bespoke solution. I have seen a site running an approach like this and I have put together an pdf as a rough guide to what I want. It uses images as an example - but I would expect that I would need a product based solution. I am interested in what back end reporting is available for users - for example dispatch labels, invoice generation etc. sales reports, stock levels. If you have a better solution them I would be happy to consider it, but it has to run over the sites as I dont want to have to integrate it all into multiple site designs. I would like the option to use a simple link to PayPal and also to be able to link to a merchant account.