Integrated Shopping Cart and Checkout

I am trying to accomplish a very very basic e-commerce aspect to a website I am building for a friend and its proving to be very difficult to get the right combination of things in the right order. What I want to accomplish is the following:

A) To design the catalogue myself. This means, without any interference from any 3rd party software I need to be able to present the markup, images, etc. for the catalogue. The only pieces I expect to use from shopping cart software is a button that adds it to a cart, and a way to send dropdown selection info such as t-shirt color, size, etc. and a price.

B) To integrate the shopping cart so that the 'cart' is fully under my control. I need to be able to control everything about how it looks, where it is presented on the page, what window its in, everything. The only thing I expect to know nothing about is how it works, ie. how it is able to add products, edit products, show subtotals, link the checkout, etc. In fact it need not have all that, as long as it shows what the user has so far, im happy.

C) Finally, I need the 'checkout' to be integrated in the same way as I described #2 above. I need to control every aspect of its look and feel and positioning. It cannot be a second website, all the magic needs to happen on my website, at my disgression, with my look and feel. The only thing about the checkout I expect to know nothing about is how it actually processes the payment, sends the order email, etc.

With all that in mind, (fully custom catalogue, custom look/feel cart on site, custom look/feel checkout on site) I thought it would be simple to find a way to do this, to find a company or open source software that would give me the functionality of the cart and checkout with none of its own dictates about where to put them or how to make them look, etc. But the more I look into this the more I get lost - I cannot seem to find what I am looking for. If there is something out there like what I need and its terribly obvious, then I must be having a bad day

Thanks for any help, its appreciated in advance!

If you find something that works like that and it's written in PHP be sure to post back and let me know.

EDIT: And if you don't, let me know how much it's worth to you.