I've had a couple of occasions where I have needed a refund or replacement from Amazon, and I have to say that they have always had the finest attitude that I have ever encountered from any retailer.
Call the customer services, explain the situation, and more likely than not they will give you a postage paid label, that will get your item back to them and track it into their system, and pay you the refund.
Dont try to send stuff without a label, they handle too many packages and it will/may end up lost.
Amazons return service standard has caused me to very significantly increase the amount I buy from them.