No matter what you do with regards to postage you will receive complaints.

I have received complaints when I have combined postage and reduced it down to less than what the postage is for 1 item - and charged less than it actually cost to post the item. I have received a DSR of 1 for postage cost in this situation as well. (Yet another aspect of the feedback system that doesn't work). Just goes to show no matter what you do some buyers will never be happy. Thankfully their are in the minority. I don't think many buyers know how much postage costs, and given the cost of registered mail, tracking etc isn't included on the AP postage labels and satchels don't have prices printed on them how are buyers toknow what is reasonable. 

You need to do what works best for you. If you have a lot of buyers that purchase multiple items and you offer a combined postage discount and decide to incorporate part of the postage cost into the item price, this will result in them paying more for their items in total. If you typically sell single items then it won't make much difference except your FVF will increase slightly.