Not certain I agree about the payment part. Why not leave it up to the DM receiver to decide whether / not they want to charge for their time, and to respond accordingly?
I mant the platform should charge to let the DM reach but yes the DM receiver can also set the charge beforehand or if he is a small time player then he can be allotted some default fee or maybe he can receive the DM freely also. This can be done by fone tuning later