I don't think that sending one letter can be construed as harassment. My dictionary defines harassment as:
"the act of tormenting by continued persistent attacks and criticism"
Harassment: General
Under the United States Code Title 18 Subsection 1514(c)1. Harassment is defined as "a course of conduct directed at a specific person that causes substantial emotional distress in such a person and serves no legitimate purpose".
Modern Penal Code Definition
The Modern Penal Code section 250.4(MPC) defines harassment as a petty misdemeanor if with purpose is to harass another, he: 1) makes a phone call without a legitimate purpose; or 2) insults, taunts or challenges another in a manner likely to provoke violent or disorderly response; or 3) calls at inconvenient hours or in offensive language; or 4) subjects another to offensive touching; or 5) engages in any other course of alarming conduct serving no legitimate purpose of the actor.