I understand it as χαρις replacing χαρις (ordinary meaning of αντι), the idea being that there’s an inexhaustible supply of χαρις, constantly being replenished. That accords with the imagery of a πληρωμα, a plenum: it’s full, and remains full however much is taken from it (cf. the ocean). What it’s full of is, of course, χαρις (and truth): 14 & 17.
It all seems pretty straightforward. Cf. Ov.AA3.90 mille licet sumant, deperit inde nihil.