The wording of the question should be, "To whom should you send the letter of commendation?".
Anyone who has done you a kindness, given you a gift , or to whose home you have been a guest.
Cover letter
First Class Mail is delivered in three to five days.
Personally, I would not. If you sent a first letter of thanks, it is enough. You do not want to appear redundant and sort of a kiss-up (I could have phrased that better). If they called you back for...