A commit is just that a commitment to buy "x" amount of bandwidth.

If you have a commit for 500Mbps at $1 a Mbps on the 95th then you will pay at least $500 a month regardless of how little you use. If you end up using 1Gbps then you will pay $1000 a month.

As far as I'm aware there are no extra charges beyond the price for the commit and any MRC (Monthly Recurring Charges) your account may have. Though my boss takes care of that paperwork so I might be mistaken.

I'm generally right in the middle of said paperwork, and you've got it right. The main caveat is that overage cost is often (though not always) higher than the commit.

(The other caveat is I've never seen $1/mbps transit, if you find it, let me know. :))