shopping cart for wordpress

I would like to add a shopping cart to my site [login to view URL] (dev site). The shopping cart needs to be able to add multiple products, allow customers to keep shopping on the site and then checkout, coupons, multiple shipping rates, and integrate with paypal. I dont mind if you have a free shopping cart plugin, want to custom code a shopping cart, just let me know your plan when you bid. I also have a couple of other minor tweaks: 1. top nav bar - highlighted tab does not quite line up 2. testimonials font size slightly too big 3. would like to have a tab option in the wp-admin where the theme owner can easily change the main body content font of the theme