If this is what you got it's legit:
Dear Old Toad,
As a thank you for being a former MobileMe member, you received a 20 GB complimentary storage upgrade when you moved to iCloud. Your upgrade expires on September 30, 2013.
When it expires, your iCloud storage will be automatically adjusted to the free 5 GB plan. You are currently using 555.92 MB of storage.
For more information, see this article
The iCloud Team
We've all been given free 20 GB storage until September 30. Go to your System/iCloud preference pane, click on the Manage button, then on the Change Storeage Plan button and sign up for the additional 20 GB of storage (at a cost of $40/year).
You can see how much storage is being used by each category when you click on the Manage button.
OT