UPS ground is fine for Li-ion cells
problem with USPS is they ship priority mail by air and that is where the problem is. air shipping limits USPS to 3 batteries
POSTAL LAW:
Domestic — Secondary Lithium-ion (rechargeable) Cells and Batteries:
Installed in equipment, packed with equipment, or mailed without equipment (individual batteries):
The lithium content must not exceed 20 Wh (Watt-hour rating) per cell.
The total aggregate lithium...
shipping more than 3 18650's in a flat rate box is illegal
check with your post office there are signs all over the place about lithium ion batteries.
YOU CAN NOT SHIP MORE THAN THREE INDIVIDUAL CELLS AT A TIME