Yes you can send lithium ion batteries through USPS mail.
You can send all types of batteries through the USPS mail except for larger batteries such as car batteries, lawnmower batteries, motorcycle batteries, toy car batteries etc.
But small lithium Ion Batteries and other small batteries such as for toys, remote controls, flash lights clocks, radios etc can all be sent through USPS mail.
I've had packages sent to me through USPS that contained Lithium Ion Batteries as well as other small batteries and it's never been an issue.
Just don't send the larger batteries such as the car batteries, lawn mower batteries, motorcycle batteries, golf cart batteries etc and they will accept those smaller batteries as long as they are not any of the listed above batteries.