I am a very very patient bargain hunter. Over the course of almost 3 years I've been a member here, I have kept bidding on items until I got it for the price I was willing to pay. Some items took me over a year to get.
One of the things I like to do is a general search for a specific item and then I go to each link. If a seller has more than 1 item and he/she offers combined shipping, then hey, you've just got yourself a returning customer. I really hate it when I see sellers charge too much for shipping. I understand you are trying to make money, but also realize that you could also be losing out on not only sales, but a valuable customer.