PayPal Custom Button

[baltimate]

Please forgive me if i posted in the wrong area,
was wondering if anyone can help me.
I need to make an paypal add to cart button,
but i need it to calculated the first
item at a fixed postage rate, and then if a second or third item is added it needs to calculated those at half postage rate of the first item,.

first item: £4.00 postage
second item: 2.00 postage
third item: 2.00 postage
fourth: etc

any help? seem a bit tough for me.
Thanks
Anthony