The problem is, in order to use the Paypal free returns offer, the buyer would have had to register wih Paypal prior to purchasing and request the return through Paypal so that hey have records of the return.  Firstly, I don't see any indication that the buyer registered for this service, given that you had to point it out to her.  Secondly, if you refund using Send Money, there is every chance Paypal won't recognise the refund as such and will refund her a second time.