Question to traffic limitation

[DaSilva]

I have a question to the traffic limitation for a domain / customer:

What happens if I (as a reseller) give a customer 1GB traffic in the domain settings and he has more traffic this month than that limitation?
The best thing would be a e-mail to him and to me that his traffic limitation was exceeded with the amount of whole traffic in this month from his domain.
Is this possible and what must I do to achieve this?

Thanks in advance.