Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

prepay
     v : pay for something before receiving it
     [also: {prepaid}]