The Monthly charges section of your bill only shows recurring monthly service charges and credits. When a promotion lasts only 1 month, it's not a recurring credit, so it belongs on a different section of your bill.
You can find the details of your one-month promotion in the Activity since last bill section of the bill.
Heads up: If you cancel your promotion before it ends, you may see the remaining promotional credit reversed on your next bill.
Let us walk you through an interactive sample bill. We'll explain some key elements that may appear on your monthly bill.