Oh, great.  Thank you thecatspjs, I'll use the eBay invoice if AusPost will ask for it.

Thank you guys for your quick responces, it helps a lot. 

I'd never use postage via eBay but I have tonnes of eBay satchels. 

I sell cheap stuff, most of the time postage costs are twice higher than the cost of the item itself, that is why I'm nuts about postage.

It's not a problem to refund the Buyer, it's the problem of not getting postage back from the AusPost.