That's awful. What you should do now is work out an amount of time you'll leave it before considering something "lost" and add it to your policies. People don't always read them, but it can help, especially if Etsy get involved. I'd give it 7 days, usually, and that's perfectly normal.
I don't understand quite why you gave her a refund at this point, though. You checked the tracking and saw yourself that it was in a post office - not lost, but in a post office and likely to be processed soon. Did you give the customer the tracking number? She could have seen this for herself, too. If she needed it - like, NEEDED it - by a certain date she should have bought it sooner or payed for faster delivery - like next day.
This was certainly not your fault that it got delayed.
See this as a learning lesson, because I find it extremely unlikely that she's going to give you the payment back if she sent you that many awful messages that fast. Add the amount of time to your policies - if customers don't read it, you can atleast refer them to it should this happen again.