It may be a custom addendum that covers increased usage, but if the 800k/mo is unusually high traffic (eg: JohnT’s nahemployment topic, which he managed to get placed on Alexa rankings), Discourse has this FAQ:
What happens if I go over the plan limits?
Your site will never stop working! We understand your site can get a lot of visitors quickly for any number of reasons, and success is something to celebrate! If your site exceeds plan limits for several months in a row, we will contact you to discuss flexible upgrade options.
~Max