We have all paid too much for postage at one time or another, and sometimes i have also underpaid postage.
So my first tip is to dig out your kitchen scales. These are not legal for trade but give you a fair idea of what your item weighs. You can pick up digital scales for less then £10, and the amount you save makes it really worth it. You can get a basic set of scales for about £4.
You can also get a postage guide at the postoffice. this gives a list of all the options and the price you are charged for each weight - invaluable. Royal Mail also do a price finder online - which is great to use for overseas countries.
Now for the price. This is very much an individual choice although i do admit i have sometimes not bid on something if i feel that the postage is unfair (8 times the amount it costs is pushing it a little i think). Many people have pointed out that it is not just the cost of the stamp - and they are right. There is also the cost of the jiffy bag/paper/mailing bag. As well as sellotape or any bubble wrap used as well.
I personally try to keep my postage prices fair. I send all my items in a jiffy bag or a mailing bag. These are new packaging materials and i also include a compliments slip (good marketing) and if the buyers has bought more than two or three items i normally send a discount card as well. All my items also have returns address printed on to stickers (with a logo) and they have an invoice sent in a clear 'documents enclosed wallet'.
If the item is big i pack this in a box. Normally for items that are used i use a mixture of new and old packaging materials. I buy a lot off ebay myself and think it is a complete waste to just throw everything in the bin. I rip up jiffy bags and use these as loose fill and sometimes i wrap them around the item as double protection. quite often when sending individual items of clothes i slip a couple of pieces of tissue paper in to keep the creases down.
Buyers appreciate their items arriving to them in a decent amount of packaging - and i agree with them. If you put an item in a used jiffy bag, send it by the cheapest option a week after they paid and charged them 8 times what it cost then they will probably feel ripped off. If you pack it well though, with proper thought and adequate packaing to protect it, send it as soon as you are able with something for them to remember you by (like a compliments slip or an invoice) then they are far more likely to return and buy more items from you in the future. It doesn't take as long as you think if you get everything you need to hand and do them all at once.
If this guide has helped then please vote for it - i very much appreciate you taking the time!