You cant! - my point was that if you sell higher quality books you can affford to provide postage discounts because the margins are better.  Click and Send charge $9.35 for a 3kg satchel, I cap postage at $6.50 thereby providing a $2.85 postage discount and don't charge any packaging or handling - Have a look at some of the serious book sellers out there -most of them are offering free or heavily discounted postage.

The way I look at it is that up until the point you charge postage the buyer is 100% happy with their purchase - if you then provide great customer service, don't gouge them on postage and wrap the item really well they remain 100% happy (most of the time). If sellers start adding little extras like handling fees then the buyers happiness level starts to fall because they think you are ripping them off. They are then inclined to not forgive anything you may have missed in your description, delays in postage etc etc and the likelihood of them returning to buy more items from you is reduced. Is the extra postage and handling fees worth it - not to me they're not.