Unfortunately there is little or nothing you can do at this point except send it in and pay for the repair. I would contact the company first and explain what happened, they might just give you a break, a discount, even free, a good company should at least do something to help.
I have been on Ebay and PP since they started and out of tens of thousands of transactions only had 2 issues, both buyers, both lied, PP sided with them but at least both were around $500, not a bad track record considering many millions in transactions. BUT, after seeing the rise in fraud, everywhere, I now only accept F&F payments unless I know the other party or they have a well established track record. For purchases I only deal with those willing to talk to me on the phone or meet in person then also check their track record.
Lesson for us all, if having something delivered you need help with have them there when it arrives, open the package before even taking it inside if a difficult task, if at all damaged have the driver there when opened as well, take pics as you go. If OK, take in and test immediately. It only takes a few minutes using input cables to test the channels and to insure the issue is in the new device. Of course the speaker cable swap works as well so can be the first step then to verify and confirm do the input test.
Rick