Another oddity I'm finding on the PP-integrated order page I'm designing: When I preview the page and click the Buy Now button for any item, PayPal correctly shows both the price of the item and the shipping+handling total.

But when I click the Add to Cart button for any item, PayPal shows only the price of the item, no shipping. Even after loading up the cart with items and then clicking Proceed To Checkout, the checkout page doesn't show shipping costs. Any idea why?