APS: All you need to do is: In the quantity box, enter the number that represents the amount you have been advised that needs to be paid;
So if the additional payment is shown as £1 and you have been advised the payment is £4.00 - enter '4' in the quantity box;
If the amount showing is exactly what you have been advised, then leave the quantity as '1'; next click 'add to cart';
Proceed through the payment section just as you did when you placed your order.
Thank you for your payment.