The 20 GB for $20 Data Plus option is only available on the $28 /mo Base Plan and $65 /mo +Phone Plan.
It’s totally normal!
On the month of activation you'll be charged as follow:
- $20 at the time of activation: correspond to the charges regarding the month of activation (please note that the monthly subscription charges are never pro-rated, regardless of the date of activation)
- $20 at the end of the month of activation: correspond to the charges regarding the next month, charged in advance