14 DAY TRIAL // There's an addition to your 'Settings' tab, a new "Tax Rules" feature that the dev team has come up with, after receiving feedback from users requesting something be done about the process for specifying taxes in the cart posts. The big problem was with those using the 'Buy Now' buttons, as they encountered the most difficult times.
The newly added feature lets users specify the default tax rates to be used when the cart posts lack the specific tax information. It comes to wholly replace the old means of doing just that, as David Wurtz, Product Manager writes in the Official Google Checkout Blog: "For those of you who rely mostly on Buy Now buttons, it is easy to charge tax on your items with these new settings in place. And for those of you who post shopping carts that already contain tax information, you can choose to use this account setting instead."
He also mentioned as a Post Scriptum that there was another method of specifying shipping and taxes in the 'Buy Now' buttons, and provided the code that would do the job for you. Just be careful to change the example's information, so that it reflects your shipping and taxes and not his.