Plastic bottle inside a plastic bag to catch any leakage, well packed in 2 or 3 inches of bubble wrap.
Remember, unless you are shipping boxed bottles by the pallet, your single 1 gallon package is going to be subjected to the abuses inflicted by freight workers under pressure to pitch freight as quickly as possible. I literally mean 'pitch'. They will throw packages even when labeled Fragile etc. and they don't give a tinker's daxx which end is up.