This is the only point at which you can purchase Super Priority processing, so if you select Standard and continue on to the payment, you will not be able to use Super Priority at all and could be waiting up to 6 months.
So, in order to purchase Super Priority, don't submit or pay for anything now, and instead, you'll need to keep checking at midnight... and if/when it is available, it will give you the option to choose between Standard and Super Priority processing before paying for the visa.