To view your invoices go to Manage > Administration > Billing > Invoices. The View Invoices page will open up with a list of your invoices:

Here is a sample image of an invoice:

If you click on the Quantity link in any row you will see a more detailed description of that particular line item (where applicable):

When you hover over the VIEW ACTIVITY HISTORY link for any user you will see more information on the user's activation history: