This is why it’s a good idea to read the offer yourself. You’re the one with $650 on the line, not the representative.
Every thing you say is making it someone else’s fault. You didn’t get a receipt, you don’t know where the package was even mailed to, you didn’t read the offer. This is $650, it isn’t buying a can of corn at the grocery store and getting one free, take a minute and check. You clearly could have saved yourself a lot of time and effort with a couple of minutes of effort.
I hear what you’re saying about the mail office, but you need to hear it too. You dropped a $650 package in the mail, you didn’t get any kind of a receipt. You didn’t keep a copy of the label so you don’t even know where you sent it to. You have no proof you ever mailed it. I would have tried to put insurance on the package, it’s worth $650!
This is the same story someone who did NOT mail it back would say. (I believe you, I’m just saying that’s why you get the receipt).