Does level
mean anything to the business?
For instance, I could imagine a situation where the levels are the unit of management. Perhaps there is a rush for sales one quarter, and the rates for each level change. Or, is there reporting by level? In these situations is would make sense to have a separate "level" table.
Another situation would be different levels for different prices of the product -- perhaps the most you sell it for, the higher the commission. Or, the commissions could be based on thresholds, so someone who has sold enough this year suddenly gets a higher commission.
In other words, there could be lots of rules around commission that go beyond the raw percentage. In that case, a "rule" table would be a necessary part of the data model (and "levels" are a particular type of rule).
On the other hand, if you don't have any such rules and the commission is always based on the person and product, then storing the percentage in the table makes a lot of sense. It is simple and understandable. It also has good performance when accessing the percentage -- which presumably happens much more often than changing it.