The best part about Azure is the ability to add resources and scale services with a few clicks of your mouse. The ease of use Azure provides also makes the service very unpredictable from a billing perspective.
The most popular Azure offers according to Microsoft are the free trial, pay as you go, enterprise agreement (EA), MSDN subscription, Bizspark, MPN, and Azure in Open. For a complete description about how Azure can be bought and paid for, check out this link on the Microsoft Azure’s website: http://azure.microsoft.com/en-us/support/legal/offer-details/
Committing with a set monitory value is better in the long run For example, an EA with Azure will get you lower resource rates
For all usage and billing invoices, use the accounts portal at: https://account.windowsazure.com/