The essential difference is that the seller advertised and charged AP rates (in the OP if you can't find it)

So the seller is already guilty of misrepresentation. She could say 'sorry, I went for the cheaper price hoping nothing went wrong'. However something went wrong, as seems to be a trope with Sendle. The seller should bite the bullet and refund. And chase up Sendle for a refund. Their choice of carrier, their responsibility.