What answer is correct to who should we send the letter of commendation or to whom should we send the letter of commendation?

Well, my friend, both answers are correct in their own way. It's all about how you prefer to phrase it. "To whom should we send the letter of commendation" is a bit more formal, while "who should we send the letter of commendation to" is a bit more casual. Just go with what feels right to you, and it will surely bring joy to the recipient either way.