I worked the window at the Post Office for many years. I must have asked customers "Does this package contain anything liquid, fragile, perishable or potentially hazardous" thousands of times.
Postal regulations are primarily for the safety of the employees and air carriers.
BATTERIES: Batteries were OK - separate them from the atomizers when mailing. The hazard is when batteries are inserted in a product (e-cig, radio, toy, etc), they could possibly overheat and potentially cause a fire. I've seen the bomb squad called for a package making a strange noise when it was actually a noise-making toy with dying batteries inside.
E-LIQUID: The regulations are for cigarettes or smokeless tobacco (i.e. chewing tobacco). Wouldn't worry too much about e-juice matching that category.
As far as the liquid: as others have posted, make sure it is well packaged (baggies are good). If a customer told me they had liquid in their package I basically asked if the container busted, would it leak outside the box. The PO employees get a little spooked when they have to handle a package that has experienced a leak - they have no idea if it is battery acid or what it is. It's been a couple of years but I think there might be a 16 oz limit.
HAZARDOUS: The PO is more concerned with flammable liquids, items that would hurt someone if a leakage occurs, etc.
The PO wants your business, and the questions are not just to keep you from mailing something, it's for protection.
Hope this helps a little.