Since Postal prices have increased I tend to be more aware of shipping costs.  Most people are very fair and others seem to try and make their profit by over charging on shipping. 
Consideration for the seller must be given as well as for the buyer!!  The seller has packing costs that the buyer needs to take into consideration such as, boxes (or envelopes) packing peanuts, bubble wrap or what ever they choose to use, tape, time in packing and the gas used to deliver the package to the post office.  Its not just the cost of the shipping that goes into the package.
Most Buyers have a set price they are willing to spend on an item and the best way to figure your cost is to add the packing and shipping costs into your set amount.  This way there are no surprises in your total when the invoice arrives.