The Collaborative International Dictionary of English v.0.48:

Prepay \Pre*pay"\, v. t. [imp. & p. p. Prepaid; p. pr. & vb.
   n. Prepaying.]
   To pay in advance, or beforehand; as, to prepay postage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prepay
    v 1: pay for something before receiving it