When you submit your MOC to Rebrickable you can choose to make them Freely Available or For Sale. Rebrickable does not provide any automatic payment/download system, instead just links to the MOC on whichever site you nominate.
To prevent excessive paid MOCs from flooding Rebrickable, MOC designers may have to pay a small once off fee to submit their MOCs. The current fees are:
Note that the fee is a once-off payment, regardless of how many views your MOCs receive. After submitting your MOCs, the site administrator will contact you to discuss payment before approving the MOC for public viewing.