The following letter can be found in the first volume of Letters of Note.

It’s somewhat reassuring to know that over a thousand years ago, human beings were frequently making drunken fools of themselves at dinner parties and waking up the next day overcome with regret. In fact, this was such a regular occurrence in one region of China that a standard form letter of apology—pictured above—was created by the area’s beautifully named “Dunhuang Bureau of Etiquette,” copies regularly signed by hungover local officials and handed, with heads bowed, to disappointed dinner hosts. A ready-made apology and acceptance for the previous night’s bad behaviour. This particular version, which even included a suggested response, was discovered in a cave in 1900 and has been dated to AD 856.
Apology
Yesterday, having drunk too much, I was intoxicated as to pass all bounds; but none of the rude and coarse language I used was uttered in a conscious state. The next morning, after hearing others speak on the subject, I realised what had happened, whereupon I was overwhelmed with confusion and ready to sink into the earth with shame. It was due to a vessel of my small capacity being filled for the nonce too full. I humbly trust that you in your wise benevolence will not condemn me for my transgression. Soon I will come to apologise in person, but meanwhile I beg to send this written communication for your kind inspection. Leaving much unsaid, I am yours respectfully.
Reply
Yesterday, Sir, while in your cups, you so far overstepped the observances of polite society as to forfeit the name of gentleman, and made me wish to have nothing more to do with you. But since you now express your shame and regret for what has occurred, I would suggest that we meet again for a friendly talk.
That's fascinating, and I love signing off with "leaving much unsaid...", I shall borrow that!
I'm guessing this was written on durable paper, something like Washi (Japanese paper), which lasts for up to two thousand years! One of the oldest letters of note and still useful.