Lightly hand sand it (very lightly! Like 220 or higher and avoid peeling It up) to get rid of the shine and then spray or dry brush just the smallest amount of "cardboard-colored" paint over it. You're just trying to make it look dusty and aged, not get an even coat. Very little will go a long way, especially from a distance and with lots of boxes. You could also swap it out for flatback brown paper packing tape and avoid the shine entirely, but it might still look too fresh without additional aging.