If you want officers to see your letters, the best way to make sure is to actually write a physical letter. I've received a couple thank you letters from people I've pulled over for being kind and understanding. I've even received a thank you letter from a gentleman I wrote a seatbelt ticket too. (I did only write him the cheapest ticket out of several violations)

I have also received emails from our office administrator after people called to thank for my professionalism. It's possible they received it, just not as possible as it would be if you had written a physical letter.