lilacben, I do that too (weigh it with the box/whatever I intend to pack it in) - but it's not the postage I get marked down on, it's the despatch time in the past - which I don't understand as I say I'll mail within 3 working days and I always do. I generally don't leave feedback when people pay, only when they confirm receipt, and have left the feedback for me.
Grasscarp, sorry misunderstood, I thought you meant a different buyer - but that's just her hard luck, she should have asked you for measurements if she was doubtful about fit. Once bought and paid for, unless you say on your site you take returns, you don't have to take it back. It's different if it's faulty or not as described - just because it doesn't fit the buyer is her tough luck, that's how eBay works.