As a footnote to the above discussion, it should be noted that eBay
often fails to combine shipping correctly according to rules entered into the eBay
web site by the seller. It is even worse with third-party applications that depend on the eBay
API. When orders are combined through eBay
or third-party applications before payment, postage discounts are generally calculated correctly. Problems occur when buyers pay for items separately or don't wait for an invoice. For that, and other reasons, free shipping is offered for most items in my eBay
store. Unfortunately, expensive items require immediate payment because one notorious UK seller creating bogus offshore accounts to buy and not pay for higher priced stamps. It is possible to combine separately-paid orders in third-party applications like SixBit, but it may not be possible on eBay
unless the buyer paid using the shopoing cart feature.