Im very confused as this has not happened to me before

[jenni200011]

need some expert advice please on what to do..I have just recived an email from paypal and thier has been a payment reversal case opened up against me.It said the payment was unauthorised and so the case has been made..The story goes.I sold an item to a lady in Tasmania and she paid me with paypal and i posted off ( think it was early april or last month by memory)..any way she received the collectable ( it was a teddy bear) and gave me exellent feedback and said she was very happy..So now all these weeks later why do i have this case,paypal says i need to prove i sent it and i cannot find my postal receipt which is so frustrating as i try keep all my tracking info ect filed..I went looking for the feedback and it seems to be gone and i cant see it anymore ..I know the purchasers email and user ID from the information paypal has sent..What can i do..If i dont prove   have sent it within seven days the money goes out of my account and ive lost everything..Im at a loss what i can do..Can Any one help me