A letter isnt cowardly.
I would imagine most customers would appreciate being informed, a letter is more professional IMO.
Phone them and you could easily spend half the night talking about allsorts of stuff; why youve got to let them go, do you know of another window cleaner, have you got his number, do you know Joe Bloggs, is Joe Bloggs any good, is this because of the down turn in the economy, I blame Tony Blair..............bla bla bla and just before my daughters getting married too.
Write a letter and be done.
It'll look good.