Kant believed that one’s duty cannot benefit one’s self interest. In the example of the store owner while it seems like the owner is an ethical and dutiful person because of keeping his prices honest it is self-serving because doing so is driving business to his store. Virtuous ethics though are concerned with doing good based on having a rational approach as well as a strong and virtuous nature.
As far as admiring someone that always acts out of duty I think it depends on the situation. The first example that comes to my mind is based on ethics at work. Using the example of someone working at a firm that gets told to cook the books, that person might consider doing so might feel as if they are acting out of duty because they are doing what their boss told them to do but per Kant because it is self-serving (the person is afraid of getting fired for not complying) they aren’t acting ethically. I would understand the internal struggle the person is going through (if they had doubts about their actions) but at the end of the day I believe if they had a strong moral compass they would not be breaking the law by listening to their boss’s orders to perform fraudulent work.
Would like a short personal response to this