im both seller and buyer on ebay and imo it is quite ok for both sides, same with paypal (fees on both are high though)
i could understand couple extra quids for packaging if he is posting delicate, valuable items thorough all europe,
anyway
http://cennik.posta.sk/ says that 1kg, registered, large envelope, to uk will cost €8.70 with €100 insurance
so he may have his reason to charge 8 quids, but increasing total after receiving payment would be deal breaker for me
i had two situations like that with postage to Sri Lanka and Kenya, but i see no reason to charge buyer more for that,
it was me that set the price after all