Here's a bibliography I'm compiling on Letter Carriers, as they relate to early Christianity, early Judaism and the Graeco-Roman world. Feel free to add any items I've missed.
Botha, Pieter. “The Verbal Art of the Pauline Letters: Rhetoric, Performance and Presence” in Rhetoric and the New Testament: Essays from the 1992 Heidelberg Conference, edited by Stanley Porter and T. H. Olbricht (Sheffield: Sheffield Academic Press, 1993) 409-428.
Epp, Eldon Jay “New Testament Papyrus Manuscripts and Letter Carrying in Greco-Roman Times,” in The Future of Early Christianity: Essays in Honor of Helmut Koester, Ed. Birger A. Pearson (Minneapolis: Fortress Press, 1991), 35-56.
Head, Peter M. “Letter Carriers in the Ancient Jewish Epistolary Material” in Jewish and Christian Scripture as Artifact and Canon Eds. C.A. Evans & H.D. Zacharias LNTS 70; (London: T & T Clark, 2009), 203-219.
Head, Peter. “Named Letter Carriers among the Oxyrhynchus Papyri” Journal for the Study of the New Testament 31.3 (2009): 279-299.
Keyes, C. W. “The Greek Letter of Introduction,” AJP 56 (1935), 28-44.
Llewelyn, S. R. “The Christian Letters of Recommendation”, NewDocs, 8:170.
Mcquire, M. “Letters and Letter Carriers in Christian Antiquity,” CW 53 (1960): 148-53, 184-85.
Mitchell, Margaret M. “New Testament Envoys in the Context of Greco-Roman Diplomatic and Epistolary Conventions: The Example of Timothy and Titus.” JBL 111 (1992): 641-662.
Murphy-O’Connor, J. Paul the Letter-Writer: His World, His Options, His Skills Minnesota: The Liturgical Press, 1995.
Richards, E. Randolph. Paul and First-Century Letter Writing: Secretaries, Composition and Collection. Illinois: IVP, 2004.