A bonus is more like a reward for service that doesn't vary as much as commission. For example if I say I got a $5000 bonus, it means I can only get 5000 dollars extra.
Commission on the other hand depends on how much a person sells in a day. If I get 30% commission and I sell 300 dollars worth of stuff that day, I get my daily wage plus 30% of 300.
Hope this helped!