Thanks again Jason. I think I'm getting it.
The order summary and total on the cart.php will not include any shipping charge.
The customer then goes to paypal by clicking the checkout button and confirms their account details.
They are then redirected to the order confirmation page which shows the shipping charge, determined by what the delivery address is in the purchaser's Paypal account.
Therefore the total on the cart page and the total on the confirm page are two different amounts.
Have I got the general concept right now?