I charge for postage cost (estimation...usually i get it wrong, one way or another) And if i require any special materials to send an item in i include a cover charge for that. the postage fee on ebay is for postage and packaging, and you can rightfully charge for costs incured to do this.
If the fee is stated and you buy the object you are also agreeing to the p&p fee.
If the fee is stated and you buy the object you are also agreeing to the p&p fee.