I'm going to do something unconventional here and actually try to give a response that may help someone who has an issue like this one get it resolved.
If Customer Service is not OKing a return, explain to them immediately that you intend to proceed with a chargeback on your credit card. Tell them that you have already spoken to the CC company, and they have indicated that you have a valid case for the chargeback. Tell them that you will hold off on proceeding with the chargeback process if they can agree to take back the item.
If the agent pushes back, respectfully ask to speak to a manager. They'll likely tell you that the manager has no authority to do anything more than they can do. Tell them that you understand that, but you'd still like to speak with them. You'll likely have to loop through this conversation 3, 4, 5 times. Even once they escalate the call, you can't be certain whether you're talking to a manager or a peer. In less organized call centers, reps will sometimes just pass you off to another agent who will claim to be their superior, but it's worth a try.
Call during normal business hours. You're more likely to reach the domestic call centers.
Document the time of each call you make, the name of the rep you speak with, and the name of any other people they transfer you to. Also ask what department they're in and note that as well.
If, after 3, 4 or 5 calls (however many you can endure), you still have not obtained the desired result, compose a letter to a high level executive (not the CEO; maybe someone like the COO or a senior VP). Find their contact info on the Web. Make the letter concise, respectful and civil. Include a log of your calls. Send it registered, return receipt. One suggestion found
here mentions looking for executive's contact information in press releases.
In addition, you can call the executive's office directly. They employ reps in these offices to address customer service issues, and you'll probably be routed directly to them. They usually have a bit more ability to try and get your issue resolved.
As one almost always snares more flies with honey, be cordial but firm in all communication. Keep your voice calm and pleasant, and treat the situation and reason for your call as something you're doing to help them as much as you.
I support a lot of customer service folks, and having climbed the ladder through a call center CS environment, I'm a little aware of what works and what doesn't.
good luck!