Good question! it really varies. I try to strike a balance as to the time I have to put in and supplies, yet try to keep it reasonable for the buyer. For instance, if it's a really small light package of earrings. The least that can be charged via paypal for 1 oz. package is 1.48 which includes delivery confirmation.
Other times I up the price of my item and include shipping that saves me the most but will still get it there in a timely fashion. I always try to recycle my packaging which also saves.