Sending email - Quota

I know that Creator allows 500 emails to be sent per day. If I need more than that, I know that I have the option to buy more. However, currently, I do not know how many emails will be sent per day.

So I would like to know how you billing works.
~ If I reach 500 emails in one day, does it just stop sending any more?
~ Is it possible to have the system just charge me the $20/500 emails when the quota is reached?
~ Do I have to purchase a few thousand just in case it happens to go over the limit?

It would be great if you have the option to automatically charge if a limit is reached. For example:
500 emails per day is free,
If the app goes over that, I have the option to say charge me $20 for each set of 500 additional emails - to a limit of xxx (maybe 5000 emails max per day is all that I am willing to pay for).

Please clear this up for me.

Thank you.