Return router

I returned the router using the supplied USPS label. I have a tracking number. The router was received by the " return agent" the next day. I received another letter, " final notice" I owed $159 for unperturbed equipment. The csa I just talked to tried to tell me to check with the post office blah blah blah. I am EXTREMELY frustrated with the situation. I feel like it's a pretty safe bet that this equipment is in a pile somewhere and the low bid contractor may eventually find an employee to work long enough to find it. Until then, I should just accept my credit getting dinged because of this? How about a resolution near term?