€5 for a letter is pretty cheap by comparison with a bank. But yes, ring them and tell them you don't want to receive any more such letters.
Many moons ago, I got a phone call from my local Bank of Ireland branch manager asking me to come in and "review" my situation, and the next day found a £50 "consultation fee" debited to my loan account without so much as a whisper. Needless to say, I had it refunded immediately.