As a seller sometimes things do go wrong and not always your fault. Sometimes it's a glitch but as the seller you cop it. I listed an engine part for $20 buy now and somehow the postage got calculated at $24 or $25. It's not a preset amount I use (doesn't match the price of either a 3 or a 5kg satchel) and the article weighed more than 5kg anyway. Someone bought and paid and when I saw the PayPal transaction and total payment of $44 I thought something is up here, somehow that postage price was listed but was clearly wrong, even more wrong since the PayPal address was in WA (from qld the postage would have been close to $70). 

Maybe the buyers eBay address was closer and it calculated different. Maybe the system had a glitch. Who knows. 

But should I have been expected to honour that sale? Pay $70 postage plus $4.40 eBay fees plus $2 PayPal fees for a total income of $38 net? I did what any sensible seller does. Contact the buyer and cancel the sale.