I think it is because you trash the folder named Kindle
So it does not exist when you want to copy to it.
Try to recreate the folder before the copy action.
I was not able to get the “This USB device Kindle is attached” trigger to work on my system, but the “This volume Kindle is mounted” trigger does work.
It should not be recursive. You only need to trash the files/folders that are directly in the folder. Their (recursive) contents will go with them to the trash.