Return package back to AT&T got stolen and replaced with a boogus old phone, now AT&T wants me to proof i did ship it back!!!

I ordered a new iPhone for my mom and decided to return it back (unused) to get a different color.  AT&T provided me with the prepaid return shipping label and I packed everything (same package box, same packing material) and dropped it off at USPS, but later I learned from AT&T that an old Motorola phone is in the returned box, not iPhone, craziest thing ever happened to me, and now I am being left alone and trying to fight for my innocence and with proof that I did ship the iPhone back.  it is unbelievable, and no any additional help, not even some type of investigation. 

If you could accuse me didn't send back the iPhone, I also can accuse your return center and supplier/shipper that someone has stolen the phone and switched it.

I have been with AT&T for over 10+ years and this is how I got treated, seriously thinking about taking all phones under the family account to somewhere else that appreciates the loyalty and business.

v