This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
If you want to see a spirited flame war, ask this question on a.m.o.e. Since
your basic choices are to charge more, less, or exactly your cost of
postage, you have to decide for yourself how to market your auctions.
However you decide, make sure it is clearly spelled out in your auction
listing. If you are unable to put an exact figure in your listing, give
enough information so the buyer can request a shipping price or look it up