Lithium-ion batteries are classified as dangerous goods and, as such, require special labeling due to the carriers' strict transportation regulations. This is especially true for mophie-branded power products.
When returning products both domestically and internationally for either a refund or a warranty, be sure to inquire with your postal carrier if a UN 3481 sticker is required on the box. An example is below.
US only: If you used the shipping box with which you received your replacement, the label is already affixed. If returning a purchase for a refund, you will need to add the sticker to the box. Your local post office will provide those stickers.
International: You will need to acquire the sticker to affix to the return envelope. Your local post office will provide those stickers.