If you do something commendably, you do it in a way that makes other people want to praise you. You worked hard, or you were kind, or you stayed honest when that was not easy — and people can see it. The word tells you HOW someone did a thing, not what kind of person they are: “She handled the problem commendably.” Teachers, bosses, and news writers like it; friends in a café almost never say it. They would just say “You did really well.” It feels a little formal, a little like a school report. Though actually, you can also use it with a small smile, when you are a bit surprised that someone did better than you expected.