Hi, I travel with 3-D cookies A TON - whenever I teach classes, and my advice is as follows . . .
First, DO NOT wrap each cookie in bubble wrap. More damage is likely to be done in the unwrapping process than in anything else. Simply line a box with non-skid shelf-liner material and tape it in place in the box. Then arrange these cookies on their sides, using some folded up paper towel at the top of the letter to prop up the letter. The letter should essentially be running parallel to the box bottom and not leaning on anything. If you allow the top of the letter to rest on the bottom of the box, that added pressure is likely to cause the letter to snap off the base. If the letters should wobble side to side, again tuck some folded paper towel under them to keep them from doing so. The shelf-liner will do a good job of keeping the cookies from knocking into each other if she should come to a sudden stop while driving, just so long as it too is taped into the box and cannot slide, and you leave an inch or more between cookies. Then place the box flat on the floor of the car and again wedge it with bath towels or cloths so it cannot move if she stops all of a sudden.
Even with these precautions, some breakage may occur, so make extras (charge for them) and leave her with a repair kit and instructions. Better yet, when I had my bakery, I'd insist on delivering something like this (again, charge for all of the drive time) to make sure that everything arrived safe and sound, and to address any breakage myself.
But again, the biggest piece of advice I have is NOT to overpack and wrap them, because unwrapping, especially by novices, is bound to create more trouble.
I have traveled thousands of miles, across the world and back, with cookies packaged this way with little to no damage. But, then again, I am the only one who handles the cookies.