Best thing to do is find a nice bit of tarmac outside and pull the pin out. Leave it for an hour or so to cool down.
Removes the risk of a still active generator going off in transit, especially if you don't have the correct containers.
This of course is only for generators that are time ex. All the new ones i've ordered have come in metal cannisters filled with fire retardant kitty litter material.
There's normally storage instructions on the sticker on the canister.