OK, I have to admit that I am the technical type, so finding the bill was just as difficult this time, as it was last time.
But no more! This time I stumbled across a great article at Microsoft Office Support. It actually tells you how to find it, step by step. Please read it your self at http://office.microsoft.com/en-us/office365-suite-help/view-or-print-your-bill-HA102851100.aspx