First mistake was sending a replacement especially using registered post. If the first one sent did not have a big enough profit margin to warrant the use of tracked posting then it is unlikely it made economic sense to send a replacement at a much higher cost.

You would have been far better off just refunding in full and blocking her so she couldn't buy from you again.

If you are certain she is going to send you back rubbish why would you pay for that return? Just refund and don't ask for it back.

If you feel strongly enough about it report the buyer to ACORN for perpetrating fraud on line.