The media storage limit is not a hard limit. If you exceed the limit for a few days, your server will not be suspended or stopped from uploading media.
If you exceed the limit, you will be contacted and asked to upgrade your plan, and if there is no response, the upgrade will happen automatically at the next billing cycle.
Masto.host has in place an automatic remote cache deletion. Please read for details.
Media storage prices cover no more than 2 GB of monthly download transfers per GB of storage.