There are many carts available that integrate with Google Checkout or Paypal, but not many do both, and out of those, how many are free?
The question could arise as to why use this cart, or any other, if Google Checkout or Paypal already allows you to create cart buttons. Well, both services have benefits and downsides.
*Google Checkout gives a wide array of options, but it doesn't allow shipping to places as close as Canada! Or if you want to add handling to your shipping prices, there is no place to put it. Paypal seems friendlier, but their biggest drawback is they don't do carrier-calculated shipping! Ebay users may find this odd since shipping is calculated in Ebay when using Paypal, however, Ebay is actually calculating the shipping, not Paypal.
Fatfreecart.com is a simple cart solution, that is free, but also provides a paid version (ONLY $5 per month!).
In its free form: fatfreecart.com allows you to add handling, among other things, and allows for multiple options. Google Checkout users may have run into the multiple options problem, but with this cart, you are given size and color.
The big downside to the free version of this cart is that you must use flat rates.
In its paid form: fatfreecart.com gives many more options, including tax by location, calculated shipping, VAT, and secure digital downloads (Google Checkout allows for digital downloads, but doesn't make them secure, that's up to you).
This is cheaper than most any cart you would find for these two services, and they do provide support via email and phone. I have not tested their support, but I have used carts without phone support and it can be the worst time of your life when in that situation.
The site with a quick demo: http://www.fatfreecart.com/
Here is the easy way to generate the code for the free cart: http://www.fatfreecart.com/code_gen.php
Here is the site to purchase the pro version of the cart: http://www.e-junkie.com/