The system only knows to charge what was initially agreed upon. A work around we've seen has the photographer asking the client to send in a new PhotoSesh request for the extra 30 min, hour, etc. But you can also simply let us know directly at email@example.com.
Then we can process the extra time from our end, if you and the client had already discussed and confirmed it.
An upcoming update (coming soon) will allow photographers to add extra time to the gig, which would be the solution here.