{To commend ourselves?} (heautous sunistanein?). Late
(_Koin(825f)) form of sunist(886d)i, to place one with another, to
introduce, to commend. Paul is sensitive over praising himself,
though his enemies compelled him to do it. {Epistles of
commendation} (sustatik(936e) epistol(936e)). Late verbal adjective
from sunist(886d)i and often in the papyri and in just this sense.
In the genitive case here after chr(8869)zomen. Such letters were
common as seen in the papyri (Deissmann, _Light from the Ancient
East_, p. 226). N.T. examples of commending individuals by
letters occur in  Ac 15:25f.  18:27  (Apollos),  1Co 16:10f. 
(Timothy);  Ro 16:1  (Phoebe with the verb sunist(886d)i);  Col
4:10  (Mark);  2Co 8:22f.  (Titus and his companion).