If you're mailing a toy or gift that uses batteries, take the batteries out and wrap them separately in the box. If the post office hears ticking, they'll have to bring in the bomb squad to determine if your package is a "letter bomb".
Also bear in mind that, no matter how many FRAGILE stickers you put around your package, the post office is going to toss your package around in their bags and trucks. There's a good chance that, if the batteries are installed while the package is in transit, the gift may turn on and wear down the batteries during the few days it will be in transit.