Mrs C - don't refund her. The position is either that it's gone missing in the post, in which case RM will refund YOU and you can then let her have the money back - or if she doesn't want it when it does arrive, eBay will expect her to mail it back to you (or refuse to accept it) and only when YOU get it back in the same condition are you obliged to refund it. Otherwise she could keep it and have her money back too, in the worst case scenario.
I would say that you have taken eBay advice, it's not yet 15 working days, and then you are reporting to RM as missing and she will have to wait while they investigate what's happened to it - they may even find it.
If she gives you bad feedback you do have the opportunity to write a (very) short note in your defence (i.e. item went missing in post, I kept you informed)
This has happened to me and it did take several weeks to resolve, but it's not your fault so don't feel guilty. You can't refund something if you haven't had it back.