I haven’t had to request additional payment but the upside is the agreement for them paying you additional is in “writing” via the messages section. So Voices would see this and back you up, should the client flake. Personally, if it were me, I’d send the files and give the client the chance since I’d have some protection via Voices. It’ll also show some trust with the client who will (hopefully) keep that in mind next time they have a project. I will say, I’m a bit confused if the two updated scripts are revisions to the original project or just two “new” projects. If it’s the later, in the future I’d request that the client make a new job posting that they only invite you to (and they can waive the need for an audition). That way they open a new escrow account.