Shipping not showing in cart or checkout

I have a problem that keeps cropping up. There is no Shipping being added to my cart.php or checkout.php pages. I managed to solve this problem a while back but can't remember how I did it. Basically I have a floatVal of £3.00 on my cart.php page which shows until the person puts in their postcode (Zip Code) in the checkout.php and then it will change if outside the UK. However, the floatVal doesn't even show up any longer and basically all orders coming through are coming through with no postage added. So, for instance, on the cart.php page if someone placed an order for 1 programme, even before they entered their postcode there would be a value of £3 added for shipping. If they entered GB for the UK in the checkout.php page then it would stay at £3.00 but I don't even get that option any longer. It is just the price of the programme and no shipping, so I have to go back to them and say there is a problem and would they mind paying the extra £3 which, as you can imagine, is not good for customer relations. A link to the cart is www.oldtheatreprogrammes.co.uk/cart.php - if you wish to try a test buy you could go to www.oldtheatreprogrammes.co.uk/TEST.php where there is the button to pay for a £1 programme, which of course you will not have to take through to the final paypal screen, as you will see the £3.00 shipping does not show up