Yes as already said above, stick to your guns you are 100% in the right.
These types of 'ebay idiots' iv'e come across countless times, they complain about the price like as if it's somehow your fault and they blame you for it. When they bid they agree to the full cost along with the very clearly stated postage cost, nobody forced them to place their bid / or buy it.
And yes these types of idiots usually don't have any logic, that part seems to be missing inside their little brains and they are more likely to leave you neutral / negative feedback because they're a bit thick in the head, but you can easily get ebay to remove it. Iv'e seen many other sellers feedback's in the past from these divs leaving neg saying things like "I paid too much for this".. or "the postage only cost £0.96 but I paid £1.10"
I don't care if the postage only cost £0.96, if I charge £4.99 then that's what the buyer pays, I could have any kinds of costs involved in that price such as packaging costs, car fuel, parking etc. And don't be afraid to give them a piece of your mind about this and let them know they're being silly ( without insulting them ), you're fully in the right and shouldn't be afraid of their feedback response.
Chavs are easily startled, but they will soon be back, and in greater numbers