At the save order of execution, you're in the wrong place.
After the triggers are fired in Step 8, the parent object's roll-up summary fields are changed in Step 18.
You'll need to put a trigger on the parent object if you want to react to the update of your roll-up summary field. If I understand your requirement properly, you might be able to create a formula field on the Commission object that references the Contact's roll-up summary field.
Hope this helps!
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