Should I edit a cost rule or add a new one?

Add a new version when the cost really changed, and correct a version only when its value was wrong. A new version with its own start date ends the old one the day before. So your past keeps the old value, and only the days from the new date change.

Correcting rewrites the past. That is right for a typo and wrong for a price rise. Say your supplier raises a price and you type it as a correction. Then every order you ever shipped looks more expensive, and last quarter's profit changes in front of you.

Each page offers both choices side by side. Logistics asks From when these prices count: New prices from adds a version, and Correct the prices since fixes the one you have open. Transaction fees has New fee from and Correct since, and Products has New cost from and Correct the cost since. On Overhead, changing the amount of a running cost asks Change applies from: a later day starts a new cost, and the cost's own start is a correction, which the panel says. Add another rate card, on Shipping, is something else: it makes a second rate card, and both charge your orders.

Deleting a version is safer than it sounds, because nouz repairs the gap it leaves. And a cost that simply stops, like a warehouse you leave, should get an end date instead. Every order before that date then keeps its cost.