Just a further note about boxes. I have done this often, especially when returning from the US with things I've bought. I quite often buy snacks, sauces, wine, etc. Put these in your suitcase and I can almost guarantee that the US authorities will open your suitcase and take a look. And damage the suitcase in the process, for which you have no redress. Put this stuff in a box and - who cares! If they open it, they just tape it up again and you will throw the box away anyway once you get home.
All this works best if you are not relying on public transport at either airport. And it is inconvenient loading a box onto the Hertz shuttle when you've dropped off a hire car.