A commission rule is a rate with a reach: a name, a rate %, and optionally the category and the seller tier it applies to. A rule with neither is the house rule — it catches everything.
When a seller is chosen on an order, the rule is chosen for you, most specific first: a rule naming the seller's tier beats the house rule, and a rule naming both the tier and the category beats both. The order shows the rule it was priced by, read-only — commission is never typed by hand and never haggled on the order form. Write the rules once, as standing configuration, and every order inherits the same arithmetic. Changing a rule changes future orders; orders already recorded keep the commission they were recorded with.