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I'd call speak to a customer service rep. Have some reason they can relate to for being late and as you can see the account is in good standing. Most mortgage companies and cc forgive one late if the others are not marked I dont think they would have a vendetta for you. One more letter is only a stamp if you decide to write, but I'd go for the person to person mode.
It can't hurt to ask.. call Customer service ask them... send letter's to exec's... The worst they can say is no and you are in the spot you are now... I felt like I boarderline harassed some of my creditors and due to my stalker-ish, or persistant tendences I have raised my score 151 points in 2.5 months...