do i need to do anything special to send my radiator out for repair? anything that needs to be declared because there might be some engine coolant remaining? or can i just box it up and get it out in the mail?
yes. it would be considered "hazardous material" you would have to drain it good. wrap it in some thick garbage bags like construction garbage bags. pack it or take it to be packed. and then take it to a customer counter. example UPS,fed ex,dhl. explain how its packed so they are assured it will not leak. i dont think the post office does it. you also can not take it to a retail store like the ups store, or fed ex kinkos.
No, it's not hazardous. Just take your radiator off and flush it out with water. Wrap it in several layers of bubblewrap and, if you can, put some foam blocks around the inlet and outlet. Then I'd suggest putting the whole thing in a garbage bag before putting it in a box. I only do this so if any left voer water spills out of the radiator it won't weaken the box. Then just ship it off like you normally would.
Again, it's not hazardous and there's nothing to declare.