she can pay on paypal even w/o an account, using her own email and her CC. so that excuse is silliness-
it's obviously going to take a bit of time for a money order, but after 2 weeks, I'd send her a message and let her know if she can't either send me tracking on the money order being mailed or it arrives by (date in near future) then you will cancel the sale for non-payment. If she wishes to purcahse from you again you'd require paypal payment via her paypal acct. or she can use cc and check out through paypal w/o a paypal account- but either way, etc...
I keep other mainly for times when people want to upgrade to priority shipping- that way I can invoice them the total plus the extra $2 for shipping upgrade. I accept money orders, but I've never, in 6 years, had someone pay with one.
No need for you to pay fees on orders that are never going to be paid for. For me, 2 weeks would be way past my limit.
But of course, go with your instincts. :) We can only tell you what we would do!