How do I design packaging that survives shipping without blowing up my cost per unit?

Test before you commit, ISTA-style drop and compression tests exist for exactly this reason, and running them before your first bulk print run is far cheaper than eating return costs later, since a damaged-goods return can cost a large share of the item's value. Oversized boxes with too much void fill are a common trap: right-sizing your box is usually cheaper AND safer than padding a too-large one.