No chance of being recovered? Fat 32 deletions will leave stuff behind unless you write over the space. I think you'll be fine just copying junk files to the device until it is full if you are paranoid.However, I doubt folks at amazon would ever look or care to. They are probably just going to nuke it and send it out again.
If you are on mac/linux you can use dd to create a file of junk on the kindle until it is full, that oughtta do it. In windows... I don't know, just keep replicating some file there. I don't know anything about nand flash, but I assume that after 1 write over it, the data is gone forever.