she can sue but she won’t win.
there is possibility that she is trying to get free item by lying to you. she got the package and she is threating you and trying to pressure you into sending money quickly before UPS makes decision.
do not budge. you are doing everything correctly and have all the paperwork to show you are not at fault.
this is not some guy who got caught stealing $10 “first” time. I would have fired in that case as well but I wouldn’t have tried to get my money back or anything like that
but in your case it’s not just $10. it has been going on for who knows how long but it seems like just the ones you have him caught in camera is significant.
i would absolutely try to get him arrested and recoup money you have lost.
as for him planned robbery, that’s going to be harder to prove so for that one you probably want to talk to an officier you will work with when you get him arrested for stealings and/or attorney and ask your lawyer what your options are and how you can proceed.