Visa Fees 2014 Cap - how much - single payment or payment done on progress

I got selected in the recent H1B lottery. My petition is in Initial Review status. After browsing through lot of discussions in the site, I could understand that the Visa Fees is structured as : Base Fee + AICWA Fee + Fraud Prevention and Detection Fee + Public law fee ( 325 + 1500 + 500 + 2000 = 4325).

I want to know when does this payment happen. Is it paid by the employer as the case progresses through InitialReview -> Approval -> Stamping ? or is the whole amount paid at once ?

Fees corresponding to H1B application is sent with your application.