Making payment once in quarter possible under QRMP ?

It is not Correct...

• Payment is not made in Form GST PMT-06: If the payment is not made within the due date, interest will be applicable from the due date of furnishing Form GST PMT-06 until the date of making payment.
• Non-filing of Form GSTR-3B: If the return in Form GSTR-3B is not filed within the due date, then interest is applicable as per the provisions of Section 50 of the Central Goods and Services Tax (CGST) Act for the tax liability, net of ITC.

For instance,

• The tax due in the first month is not paid on or before the 25th of the next month.
• The tax due in the second month is not paid on or before the 25th of the next month.
• The total tax liability for the quarter (net of ITC) reported in the Form GSTR-3B, which is not filed on or before the due date.

In this case, interest will be charged separately for the number of days delayed in the first month, second month, and in filing the quarterly GSTR-3B. 

Rate of interest- It shall be 18% of the net tax liability which remains unpaid or paid after the due date for any of the months of the quarter. Also, a late fee is applicable if there is a delay in filing the returns (GSTR-3B/GSTR-1). A late fee of Rs.50 per day is charged (Rs.20 for nil returns), subject to a maximum of Rs.5,000. However, no late fee is charged for a delay in payment of tax for the first two months of the quarter.