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on a prepaid basis (Sounds less professional than "on an advance-payment basis' or 'on a basis of advance payment' (or 'payment in advance" )  wordreference.com Denis Lebedev) | |||
| on an advance-payment basis (wordreference.com Denis Lebedev); on a basis of advance payment (Denis Lebedev); on a basis of payment in advance (Denis Lebedev) | |||
| payable upfront (oVoD); payable in advance (raf) | |||