I am directed by the President of the United States to notify [you] to expect an attempt will be made to supply Fort Sumter with provisions only, and that if such attempt be not resisted, no effort to throw in provisions, arms, or ammunition will be made without further notice or in case of an attack upon the fort.
Doesn't seem particularly insulting or provocative. A little brusque maybe, but to the point. The letter or note, though, was simply there to confirm and establish the message that the emissary was to convey. The piece of paper wasn't as important as the emissary's diplomatic skill in conveying the message. It would have been foolish for anyone to take offense.
No, the other one.