It doesn`t sound like she is going to budge regarding paying the postage,  so to resolve it as peacefully as possible I would send a message, simply saying you have investigated all other avenues trying to get the parcel to her cheaply, yet safely with no luck,  and although you understand her disappointment it may be best to mutually cancel the sale, and ask if she will agree, if she won`t then open the dispute. 

You will however will be eligible to receive feedback,

Alternatively, just go with the dispute and be done with it.

May I also suggest, as a seller do you use click & send?  what if the items were sent in two separate parcels?  would one boxed fit into a 3kg satchel?  or even a B2 Aus post box?