If I'm understanding you correctly what you need is a US Address to receive your Kindle replacement and then forward it to you in Spain. You should be able to return the defective Kindle directly without assistance.
I believe that you can use a Shipping Forwarder service like those that are talked about in this thread here:
https://www.mobileread.com/forums/sho...ping+forwarder
I don't have any experience with these services to recommend a reliable company but I'm sure some of our other International members have utilized one or two. I personally would research this carefully as you are going to be trusting them to handle your new Kindle honestly.
This is a company I sometimes see mentioned. You can at least look over their website and get an idea of the process.
http://www.shipito.com/usa-address-mail-forwarding/