There are clearly many points of interest raised in the previous cited letter (II AD). Three in particular concern interesting aspects of epistolary communication:
a) The letter is addressed to Ptolemaios' mother and sister (who it is clear have sent other letters to Ptolemaios), and yet they are apparently illiterate and Ptolemaios calls upon someone (anyone!) to read and translate this letter to/for them. He is able to presume that this would happen, although I don't see this as performed by the letter carrier, who is nowhere named or mentioned here.
b) Twice Ptolemaios refers to other letters, but both times he associates letters with personal messengers: 'You blame me through letters and through people as if I had done wrong ...' (DIA GRAMMATWN KAI DIA ANQRWPWN, lines 7-8), and 'I blame you because you enquired about me neither by words nor by letters' (OUTE DIA LOGWN OUTE DIA GRAMMATWN, lines 18-19). (Yes, he does get a bit defensive here!)
c) Ptolemaios refers to a specific previous letter that they had sent, which he claims not to have received. He realises that such a claim - I never received your previous letter - is such a commonplace, so he adds a further denial: 'No, I did not tear it up'!:

Concerning the letter which you sent to me, [I,] as [I] did [not] receive it, said: "No, by Sarapis, I did not tear it up; for I am not stupid. (lines 52-57)

For Greek text and photos: A. Bulow-Jacobsen & V.P. McCarren, ‘P. Haun. 14, P. Mich. 679, and P. Haun 15 – a re-edition’ ZPE 58 (1985), 71-79.