In Sefer Bereshit, 30:3-13 the maidservants of both Rochel and Leah are given to Yaacov in their stead to build Yaacov's house through bearing additional children. Since the text states that any offspring will be counted not only to Yaacov but to Rochel or Leah, it implies that the maidservants, Bilhah and Zilpah, were acting as agents of Rochel and Leah.
Since bearing children is life threatening for women and in light of the possibility of legal agency, it raises the question of whether a maidservant is required to risk their life for their master? Or does this reaffirm the idea that their participation was voluntary, like a legal agent, meaning a shluchah?
One added insight in connection with this is that if shlichut is present, this could resolve the question revolving around whether Bilhah and Zilpah were like Rochel and Leah, meaning of equal status or not.