Here's another angle...

The buyer is a scammer.

They have one of these stickers they got from some other seller.

Then they just overlay it onto any envelope they wish, take a pic and send it to you knowing that you will refund them the $2.50. They get a free $2.50 each time they do this to a seller, never having actually paid the fee themselves.

I say this because it simply does not make any sense that the recipient would get the fee when your return adddress was clearly on the envelope.

Not sure what checks you could do at Aust Post to find out if this buyer has really gotten this fee.