Just because you paid for airmail doesn't mean 
it went by airmail... mistakes happen.  I live 
in the US and once something I sent airmail took 
almost 6 weeks to reach the UK.

It is kind of brainless of the buyer to request 
overnight shipping on something that was already 
sent ~_~  I don't think it should be 
considered 'lost' until about 8 weeks passed, so 
the buyer will have to wait.  It's up to you if 
you want to offer any compensation if it is 
indeed lost, but I would give it much more time 
to arrive first.  Unfortunately, if the buyer is 
uninformed about the way the mail sometimes 
works and is pigheaded about the whole thing, 
you might not be able to avoid a neg.

I would calmly explain to her (without insulting 
her intelligence) that sometimes mail gets 
delayed.  This can be for many reasons - customs 
holdup, temporarily misplaced in some postal 
facility, or being sent by sea even though you 
paid for air.  It is not rare for things to take 
longer than they should, and she should not 
start considering it lost until at least 8 weeks 
have passed.  Was this sent to the US?  Because 
the US mail is being very whacky lately which 
probably will not help the process.  Over the 
past month I've had priority mail take 7-10 days 
to arrive rather than the 2-3 days it should 
be.  Very odd.  But anyway, she should go check 
out the eBay shipping forum if she doubts this - 
im sure many people can tell her that mail gets 
delayed all the time.

If she is convinced you are a scammer, then she 
probably wont believe a thing you tell her.  But 
I still wouldn't let that corner you into 
refunding $195 when only a few weeks have 
passed.