'Letters' in the Bible

Do you say that this is self-recommendation once more? Or do we need, as some do, letters of recommendation to you or from you?

Let it not seem as if I wanted to frighten you by my letters.

For they say "His letters are authoritative and forcible, but his personal presence is unimpressive, and as for eloquence, he has none."

Let such people take this into their reckoning, that whatever we are in word by our letters when absent, the same are we also in act when present.