Hi crjebay - I think it's ok to add 2 or 3 dollars to the actual postal expenses to cover packing materials. Any more than that and you will probably start losing bidders - you start becoming non-competitive and some (many?) buyers will start to feel you are charging too much.
If you feel you need to charge more, then you might consider folding the extra into your starting bid. Some sellers will even charge less than their actual shipping expenses ("Free Shipping" for example) and make it up on the price of the item. You have to take into consideration what the competition is charging for s/h.
