I think there is a strong possibility that even if she takes it back to the PO, you won't be able to claim insurance on it. All insurance companies, including the PO service, will get out of paying if they possibly can and unfortunately, you posted a prohibited item.
Even if she opened it in a PO, they could always say the packaging wasn't adequate in any case.
I don't know about anyone else, I can only speak for myself, but if a seller suggested I open a parcel in the PO, I probably would not comply.
You say you're surprised that the buyer doesn't want to solve the problem asap. I think she does.
She is suspicious because even before the item arrived, you were telling her to open it at the PO in case she needed to submit a claim. She's wondering if maybe the lid was potentially faulty and you knew it.
Now you're telling her she will get a refund only after she returns the parcel to the PO.
She isn't sure how that will work out, so she's going the sure route, through ebay. I can tell you now, I'd probably do the same.
Give up trying to persuade her to take it to the PO.
I don't see what else you can do except refund her but before doing so, perhaps ask her to send you a photo of the damage and leaking. Tell her you will issue a full refund.
Edit: Just read in another post she has already sent you a photo.
Just refund, in that case. Nothing to be gained by dilly dallying around.
