Voice123 is a marketplace that works primarily as a 'connection service' between voice actors and clients. The casting process is free to use, the only cost involved is what you, the client, pays the voice actor. When the casting process is over and, a voice actor has been selected or hired, any negotiations about payment, deadlines, and NDA are between the voice actors and clients. Voice123 is not part of the final transaction.
After the auditioning process, Voice123's system can be used as a communication tool with the voice actors. It's up to the voice actor and the client to negotiate the actual rate, possible buyouts, and residuals.
If you selected the voice actor you want to work with, send them a message letting them know they're hired.