If you are using the Basic PayPal e-commerce Widgets, it is not possible to display shipping costs. You need to add the cost of shipping to the price in the description field or include it in the total cost of your product.
Alternatively, you can check out the feature-rich, powerful Online Store. Offering a huge range of product and customer-management features, the Online Store lets you configure, among many other options, courier-shipping, discount vouchers, email notifications and a multitude of currencies.