Unless something is really heavy I've always charged a flat rate of £2.50 for each item (and £1 for each extra item if buyer wins more than one of my auctions), and state that it will be first or second class depending on the weight. I think its reasonable to ask for a little bit extra to cover other costs - packaging, petrol, parking, etc. and up until been happy that I'm not out of pocket but by the same token am not ripping people off either.
I send a load of parcels off this morning - nothing too heavy or bulky and the first class price was £1.94 and second class £1.67 (or something like that anyway!) Just seemed more expensive than usual and am considering charging a little bit more in future. Just wondered what other people charge/are happy to pay.