@ Peppermint - perhaps write a nice message back and say that you are unable to refund as the item is still in transit and, as she is aware, there are massive delays in the network which are out of anyone's control.

You could also add that not only were you unaware of her deadline, but since it will arrive after today, then it's best if she refuses the package so that it gets returned. Once you have it back, you will refund the cost of the item only, not the postage as it would be considered a change of mind return. See what happens.

I reckon the reason she's not opening a dispute is because she knows she won't win it.

Please don't be afraid of negative feedback. There are circumstances under which it can be removed, depending on what the buyer says. If it can't, you can always write a professional response that FUTURE buyers will see.