There has to be a decent shopping cart plug-in out there…

Can anybody suggest a decent shopping cart script?
Most of the stuff I see are full fledged complete solutions to make online stores or websites.

I just need something simple, like a shortcode or shopping cart gif that can be included in the product posts our company already has (instead of having to do the complete store catalog all over again). Then the values would be summed up in a checkout page, and from there they would be connected to Paypal, Worldpay, Whatever.

Can anybody suggest something simple and workable like this?

Thanks.