I only/ never want to ship in envelopes
Deactivate all boxes and activate envelopes, or activate all boxes and deactivate envelopes.
A rule is usually not necessary unless you want to enforce envelopes for a specific combination.
My custom S box runs out frequently
Keep a Hive Standard box enabled as a fallback to avoid automatic upsizing to larger boxes.
I ship testers or flyers
Mark them as negligible.
Keep rules focused on products that determine box size.