Discount not passed on to PayPal

Just discovered a rather embarrassing glitch. When you enter a valid discount code and click “Update Prices”, the discounted total is calculated and shown, but when you click “Checkout”, the full, undiscounted price comes up for the PayPal transaction.

At the moment we are doing a manual PayPal refund when requested, but we still have confused, unhappy customers. Is there any chance of a quick fix?

Many thanks,
Graham