Being charged for phone after returning back to Assurant...

I had my very first phone break.  I took all the necessary steps to send back my phone.  I put it in my mailbox the morning the mailman was coming to pick it up and had my camera on the mailman showing him take my mail that morning with the phone in it.  I get letters telling me I owe them for a phone at full price.  The company I own builds cell phone towers for T-Mobile and this is how I was treated.  There is no way for the customer to track the package once it has left your hands.  There is no usps first class tracking available for this package.  I was inside the post office with a manager for almost 2 hours trying to find out where the package was.  After spending 2 full days dealing with Assurant they still give me numbers that are not valid tracking numbers.  So I took it a step further and called consumer affairs with the usps and I was told  this ", the number provided is the tracking number I requested to trace your parcel in the USPS system, although, after checking it, the number appears as invalid. It is possible that Assurant generated a bad label. Please contact Assurant to dispute it; USPS is unable to assist further.