I ship a lot of coins in flat rate boxes. I always use filament tape (available at any office supply) to secure the box. For heavier shipments, every inch of the box is secured that way.

I also try to place the contents in a freezer bag with my business card included in case the contents are separated.

Bag + Box + filament tape = triple protection

One other thing: I never rely on the self-adhesive box (or flat-rate envelopes) to stay closed. Even for lightweight loads, that seal gets taped

I ship about 10-12 packages a week. I have been fortunate to learn from others' mistakes 80% of the time and my own mistakes 20%