Hahahaha, I have often thought the same thing about USPS! Even if I tell them "yes" when they as if there is anything liquid inside they don't do anything different to the box, lol. So weird.
I charge my single removable-battery mods via USB frequently and I have never had a problem with it. The only mod I don't do that with is my X-Cube II because SMOK specified that the USB on that mod is strictly for updating. Also, it's a dual-battery mod and I feel better charging those batteries on the charger.
As for keeping your mod in the glove compartment, I don't think it would hurt the battery. If you are worried about that you could always just take the battery out of the mod, I suppose. The only problem I have ever had with having mods in the car is that the juice gets warm and sometimes it makes the tank a little leaky. Of course I am not an expert, so maybe someone else has a better answer for you. This is just my own personal experience.