Are you sure that the postage should have cost more than the $10.75 you were quoted and ultimately paid though? For example, if it weighed less than 500g it would have gone for the standard Australia-wide parcel rate of $7.20 plus $3.50 SOD, which comes to $10.70, but once the weight exceeds 500g, the price skyrockets. Perhaps your seller quoted on a 500g parcel without realising that it weighed more and has decided to wear the extra because she's simply honouring the quote she gave you, in which case I think it's the decent thing to do to send the difference (although most people wouldn't).

Calculating postage can be a real minefield these days, unless you use prepaid satchels, especially with cubing adding huge amounts to parcels weighing in excess of 500g so as you say, it appears that she's definitely an inexperienced seller who has been badly caught out, so good on you for having the decency to cover the additional costs so that she doesn't lose on the transaction.

Like you, most of my items go via large letter for either $1.40, and those which would cost $2.10 or $3.50 I send in prepaid C5 envelopes which only cost $1.70 each in bulk, so I just charge a flat $2.10 for everything regardless of weight, and that covers the cost of the small amount of bubble-wrap or the padded bags for which I only pay 17c as well as ebay's 10% cut, so I lose nothing on my large letters regardless of their weight. I try to avoid selling items weighing more than 500g for obvious reasons.