Card collections live or die on corner condition, and a bent corner or a scuff from cards sliding around a loose box can knock a near-mint card down a full grade. Keeping cards upright, snug, and unable to shift is the whole job.
Pro tip: A cheap plastic card box with a locking lid travels far better than an open shoebox; the snug fit is what keeps cards from shifting.
Watch out: A half-empty card box lets cards slide and knock corners against each other, which is one of the most common causes of grade-reducing damage during a move.