I think a business card and thank you card would be fine. However, you may want to print this information on a business card (you can pick up a pack that would go through your printer @ Target or Walmart for cheap) in case customers passed over it in your listing, just as a disclaimer. It'd also be useful for people who receive it as a gift and wouldn't know it needs to be protected outside:
"This sculpture can be hung outside, but should be protected from weather conditions; humidity will make it oxidise quicker and might cause a green sheen over the structure as is natural for copper.
It is not designed as a toy and contains small elements, therefore it should not be hung within the reach of a child or baby."