GetShopped is a free WordPress
It is a very easy-to-setup plugin which can work with any standards compliantWordPress theme (and WordPress MU).
The plugin offers a flexible pricing structure for products with features like:
flexible coupon/discount pricing rules
product specific sales
multi-tier pricing for quantity discounts
and supports multiple payment providers like Google
GetShopped makes it possible to complete the shopping process quickly with a one-page-checkout & optionally, guest checkouts is supported.
It is easy to manage orders with the help of a powerful admin interface, print invoices, packing slips & more.
The plugin also offers lots of other features from comments to new/featured product
More Info: http://getshopped.org/