Honestly, if you can't perform a simple Minimum Bursting Test to see if your box exceeds the 14.1 kilograms per square centimeter limit, you clearly shouldn't be reimbursed for the antiques we obliterated. And did you even DO a Minimum Edge Crush Test? When's the last time you had your Edge Crush Tester calibrated?
Seriously, though -- use a new box, and use wide packing tape on it. Oh, and I'll give you an extra hint they don't mention: Waterproof what's inside.
Teshub, the storm god, is package delivery's mortal foe.
The trailers that take your stuff across the country are gigantic metal or wooden boxes that sit outside in the elements for years. Sometimes they get holes in them. Then, when it rains, they leak. Nothing made me feel guiltier than loading stuff under a leak, water trickling down over the cargo you clearly expected would stay dry during its trip. We can ask for a replacement trailer, but that backs up the whole process -- in the 20 minutes it takes to get a non-leaky one up to the door, the packages are backed up, stacked up, and basically murdering each other as the belt throws more and more at the heap.
If I've made you paranoid that you can't ship something expensive without spending two hours vacuum sealing it in layers of Kevlar, let me offer a quick tip: Buy a cheap plastic cooler. They're like 10 bucks. Put the expensive item in the cooler, put both inside a box. If you don't understand why, take a cooler out back and pound on it with a baseball bat until it breaks. You have a better chance of the bat bouncing back and breaking your nose than you do of breaking through that cooler.
This will protect your gifts from anything short of skeet shooting.
Please don't underestimate how much this thing is going to get abused -- these packages sit on a slide while hundreds of other packages push from behind. If an especially heavy package comes sliding down on top of yours, it will burst that box and flatten it in a spray of packing peanuts. If not, then I'm going to use your box to play a game of Tetris in the back of a semi (and I won't lie and say I never stood on a package to reach the top of the trailer).