I've just checked this iussue and feel the need to explain how PayPal Express works.
1. The button on the shopping cart page will be clicked by a customer.
2. He will be led to PayPal and the subtotal (without shipping costs) will be displayed in the PayPal terminal (which is correct because shipping costs aren't available yet)
3. The customer will be led back to the shop where he can process further in the checkout process.
4. The payment selection page will be skipped because the customer has already made this decision.
5. The customer clicks on the Confirm button in the last page of the checkout process. Now the final order total will be transmitted to PayPal without any interaction. The customer will not be molested with any infomation about this process. Now the transaction is complete. If the customer does not click on Confirm no transaction will be made.
6. The order amount was payed fully including shipment costs.
This workflow was predefined by PayPal and every shop which is using PayPal Express has implemented it like this.