My business is selling camcorders. I am allowed to ship, by air, a camcorder with the Li-Ion battery installed and also include up to two, new in packaging, extra batteries in the same box. I used to sell batteries on their own but I am no longer allowed to ship them by air. This has always seemed a little strange, to me.
Li-Ion batteries, on their own, seem to come in by air in their hundreds, from the Far East, and are then delivered by Royal Mail with no questions asked.