There needs to be a gap of at least 30 days between SIP orders.
If you choose to pay for your first SIP order while setting up the SIP then the first order has to be paid for just like a one time order.
If the gap between your SIP set-up date and the next SIP date is less than 30 days, your SIP order is automatically scheduled for the same date of next month.
You can track your active SIPs and upcoming SIP orders by going to SIP | STP | SWP tab.