The calculation of postage via USPS at the postal site is what you pay if you go to the post office. If you sell something via eBay and have posted the flat rate for shipping (I calculate by using my zip code here and putting in the zip code of my home town in Washington--generally as far away as I can manage on the continent) and use that as the advertised shipping cost, you can sometimes do a little better than break even. When I pay and print the label via PayPal there is usually a discount of some sort in there--sometimes only 50 cents or a dollar, sometimes several dollars. The post office usually charges the same on book rate whether it is going near or far. Parcel post is by zone. I don't know how to calculate that discount beforehand, and if a buyer wants to check on the accuracy of my rates they can see the standard cost at the USPS site. There's also a discount on the tracking fee (.14 vs .60). So I use PayPal for shipping when I can.

SRS