All too often buyers will accuse sellers of making a profit on postage costs, which some sellers do but many, such as I do not and sometimes have even made a lost.
Working out postage costs is simple - I package my items in the box or bag that it will be posted in, place on my eletric scales and make a note of the weight in kg and grammes. I will then log onto Royal Mail where I will use the Price Calculator and put in the weight, location, service (recorded, special, airmail, etc) and if applicable the dimensions. This will give me a price. I then add the actual cost of the packaging and put this up in my listings.
I encourage questions from potential buyers and if someone wants to know postage costs for their location, I simply find my PRICE CALCULATOR on the Royal Mail site and add their actual location
When buying I will ask for the actual weight of item and make a note of the location where item will be posted. I understand that valuable and fragile items need to be well protected and insured so I don't mind paying extra. But if I don't want to pay high postage costs, I simply move onto the next seller.
Simple and ultimately a common sense exercise.