I mean its not as if it's extortionate, maybe £2, but I think they agree with those terms when they buy whatever it is you're selling. Am I wrong or what? lol
the buyer shouldn't be able to tell how much you paid for shipping anyway - when you ship through paypal it says "display postage value on label", always check no, then the buyer has no clue how much it cost to ship.I always refund the difference if there is one. Sometimes I have trouble with the international shipping so I overcharge a couple of dollars on purpose, then refund the difference.
I don't sell bags though, just books but they can be REALLY heavy.