Integrate shopping cart into existing site

Hi all,
Basically at the moment my site www.alite.co.uk has a shop bolted on to a different link (/shop), but what I am looking for is a shopping cart solution which exists over the whole site and doesnt need a seperate shop section.

Are there any shopping carts which offer easy integration (i.e, just giving a shopping cart in the header and a buy it function in each of the products?)

I enjoy the flexibility and affordable scalability osCommerce provides over any other system.

If a client came to me with this request, I’d suggest to base it off osCommerce or loaded osCommerce template which:

a, sits on a dark background (that will save you a lot of stylesheet edits and file edits)

b, possibly has the right column already removed and center portion stretched properly (that saves even more edits)

Something like this:
http://www.oshelpers.com/oscommerce-templates/oscommerce-templates.php?pic=110

…then all you have to do is embed the current header (into header.php), footer (footer.php) and background image (stylesheet.css) which is fairly easy to do.