Discount postage on multiple fixed price purchase

[stuffahoy]

Hi, I've just done my first multiple fixed price listing. In the advanced listing page, you enter the postage details, but you only have an option for a price for extra item additional postage. I could send 1-3 items for the same price, but if someone wanted 4 if would be more.

Someone tried to buy two items, but it defaulted to charging him for postage twice so he backed out. How do you set ranges for postage? Is it safe to leave the extra postage field blank and just put a note in the Description?

As it was a Buy It Now scenario, does the purchaser have the option to delay payment (like auction) and I have to do something with the invoice?

Please help. Getting my head around postage is doing my head in!!