I'm just going to put this simply - if your PayPal claim is for a postage overcharge, it will not be successful, unless the seller voluntary accepts your claims and chooses to refund themselves.
PayPal provides the ability to open disputes for partial amounts, for things like (what the buyer believes to be) excess postage charges, but they only offer actual protection for items not as described, or items not received, and in both of those scenarios, it is all or nothing.
I have no idea why the seller did what they did, so I am not going to comment on that, except to suggest depending on the size of the item, a box may have been cubed by Aus Post and therefore cost significantly more to post than the seller expected.
Personal opinion - I might have been a bit disappointed to see such a large discrepancy in postage, especially when I was given to believe the (potentially fragile?) item was going to be boxed and then it turned up in a tough bag, but if the overall price was good regardless, and the item arrived in as described condition, I'm more likely to take it on the chin and move on. This is not to suggest that everyone should do as I do, rather more a statement that there are other things I'd rather put my time and energy into.
PS - I suspect you won't like to hear this, but you kinda are here to pay for the seller's fees. I wouldn't actually put it like that myself, but literally no seller makes money / profit unless all of their expenses are covered. Out of curiosity, if you are happy to pay for those costs in the item price, is there any reason why you are not happy to pay for them in the postage cost? Is it an awareness thing? Like, if it's in the item price, you're less aware that you're covering business expenses in addition to the cost of the item? Or is there some bigger principle involved, regardless of the total you actually spend, that means you prefer to have those costs in the item price?
