Kennethcole.com

I purchased a bag at kennethcole.com on Feb 1, and returned it back through USPS on Feb 12 because of the wrong size.

After 2 weeks, I contacted the customer service if my returning is in right process as I haven't received any feedback. They answered they can not assist me and refund the amount I paid for that bag just because my returning is not a traceable method. There is no mentioning on that in its return policy on back of the invoice. I have the receipt which proves I sent a package to the company, and even I attached it in the email.

But they still refuse to refund.

What shall I do?