So normally you need a crowbar in order to open a box with a lock on it, however there is a neat trick that is much easier than finding a crowbar. If you open the car door, drop the boxes next to the seat, sit in the seat, pick them up, and then drop them inside the car you can glitch your vision into the box and pick up the items inside of it like you would pick up any ole item. You can use this same process (without taking the items out of the box) to carry multiple boxes with sellable items in them and take them to a gas station to sell them. Just leave them in the car.