I need advice on a Paypal button

I am trying to add a paypal button on this site: https://www.cottagefarmorganics.co.uk – I had initially added the paypal buttons straight from Paypal.

They all worked fine – but my problem is that (the person I am doing the site for) he wanted the postage at ￡16.00 but now he wants a discount for each subsequent box in the same order which would be ￡8 or if it’s ordered locally ￡11. Is there a way that shipping could have different pricing?

Any advice please?