Way to go jeni.
I buy vehicle parts from the UK and sometimes I notice the seller has paid more in postage.
On my next purchase I send a message asking them to add another 3-5 pounds onto postage from last time.
The way it should be IMO. 
(Love Bega cheese BTW)
Throughout this thread, I have felt a bit as if I have wandered into an alternative, genteel ebay world.
One where both buyers and sellers have been keen to make adjustments after the sale, to make sure everything is fair and square, so to speak.
All I can say is I don't think the people on these boards are totally representative of the wider community of ebay users.
In my selling days, I never had any buyer refund money on those occasions where we had underestimated it. If anything, it tended to go the other way, with occasional buyers asking why an item (such as a large snowsuit) was not sent in a 500g satchel when in fact the item was too big for even a 3kg satchel.
I think the key factor really is that sellers need to be on the ball, they need to set a selling price where they have a profit margin and they need to set a postage price that at least covers costs.
If they make a huge error & would be a lot out of pocket, they probably need to opt out of going ahead with the sale. A small error they should wear.
I know a seller may make an honest mistake but a bidder might only bid because the price is right. Even a dollar or two extra might push it up over what they wanted to pay.
