Their argument, in a nutshell, is that the written texts comprising the New Testament—and specifically the gospels of Mark, Luke, and John—bridge the distance between the absent Jesus and the present audiences of the texts. The evangelists, or actually the gospel texts themselves, serve as messengers (Boten) mediating the message and making present the message's sender to the message's recipients. You can find one particularly useful statement of their thesis on p. 222: "[M]essengers are not only a link in an information chain, they also have to be understood as a medium of communication. Messengers do not just bear messages; they are also media of transmission."
All that simply to introduce the following quotation, found in the authors' introductory comments to their discussion of Luke's gospel as postal communication:
The messenger plays an important role behind the transference of information. That is, the reliability of the messenger has a prominent role in successful communication. . . . The qualities demanded here like dependability and credibility were transferred to the messenger to fulfill the purposes of the absent transmitter; nevertheless, this is to be understood not in a sense of fidelity to the letter of the message, but the fidelity to the sense intended by the sender. It is not about preserving, but about passing on directly. Therefore, the protection of the message aims as Horst Wenzel has explained, not at the authentic preservation of the message, but at the dependability of the messenger. (228, citing H. Wenzel, Hören und Sehen. Shrift und Bild. Kultur und Gedächtnis im Mittelalter [Munich: Beck, 1995], 262; my emphases)