With express checkout, the user goes to the paypal site to enter the billing and shipping info, they are then returned to your site and the shipping and tax are calculated. they have the option to confirm the transaction or not at that point.
once the tax and shipping are calculated, they are stored in a session. you can add a clear session value server behavior to the cart display page to clear the tax and shipping sessions so they are not shown.
have you run the checkout wizard multiple times?
If you run the checkout wizard multiple times, it will add multiple behaviors for looking up the shipping, this will cause the shipping rate to increase.
if you need to re run the checkout wizard, make sure you delete the existing checkout pages first.