As a Postal employee, let me say this: pack it like you expect it to get broken so that it doesn't, marking stuff fragile doesn't help (in some circles Fragile stands for "Not Broken Yet"), make sure it doesn't leak. Also stick a ship to/shipped from address card inside the box.
I've seen too many things put in a flat rate box with no padding of any kind. As soon as it's dropped (yes dropped) from a shipping container onto a belt for sorting, it splits wide open and gets sent to "Patch & Repair". That's if all the parts stay together.