Main line is post paid. The billing cycle date is not the same as the Autopay date it seems.
Checking my account it seems that it will be charged $10+tax each month.
For March it says $0 and my current balance says $10. Last night it showed $0 balance after activation.
Comparing this to my previous Prepaid line, the details listed for the plans are the same wording.
For my previous plan, I was able to access unlimited throttled data after my 250mb was used up and my balance was $0.