If you are replying to this: "Their salesmen are usually on commission," it is correct as written. Take your time and read it again. Since you didn't actually reply to that particular post, however, perhaps you were playing Grammar Nazi with someone else.
If you are replying to this: "Their salesmen are usually on commission," it is correct as written. Take your time and read it again. Since you didn't actually reply to that particular post, however, perhaps you were playing Grammar Nazi with someone else.