NONE~
Okay, seriously not true but almost.
First, consider the possibility that her mail carrier smokes (mine does) and if it has a slight smell it could just be from transit.
Assure her that you do not smoke and that the bag did not smell of smoke when it left your home. I would not agree to take the bag back and see if she files with Paypal. If she does, it is likely she will get granted the ability to return the bag to you but hopefully if you stand firm with no refund then she will just back-off.
Did your listing say "smoke-free" if not, Paypal may decide it is not SNAD.
Sorry you are going through this and post an update when you have one.
*** Ahh, you typed while I typed.
Now all you can do is sit back and wait. In the message to Paypal be clear that you don't smoke and neither does anyone else in the household. I really hope you win, but I fear you will not. If the delivery man doesn't smoke then it is likely a case of buyer's remorse.