Lithium metal (nonrechargeable) cells and batteries and lithium-ion (rechargeable) cells and batteries are mailable in limited quantities domestically via air or surface transportation when they are installed in or packed with the equipment they are intended to operate.
Here is the exact DMM section relating to shipping lithium–ion batteries via USPS within. Here is a link to the USPS Domestic Mail Manual 601. You need to look at section 10.0 Hazardous Materials. You will probably need an ORM-D label.
Stay in compliance and ensure the safe transport of your shipments with USPS lithium metal batteries â€“ forbidden for transport aboard passenger aircraft .