Tax are shipping are calculated based on the Shipping information that is entered in the checkout form.
it is not possible to calculate the tax or shipping before the checkout form is completed.
Once you fill in the checkout form, the confirm page will show the shipping and tax in the cart summary.
delete the checkout form from the checkout page.
on the cart page, set the checkout server behavior to pass a URL Variable to the checkout page:
on the bindings panel, click the plus button and select URL variable, name it "express"
on the checkout page, double click the Authenticate transaction with paypal express. for the trigger, click the lightning bolt icon and select the express URL variable.