Tax rate on interest charged on late payment

Mr. X sold goods to Mr. Y on Jun-18 

Product A 1,00,000 + Gst Rate 5 %

Product B 50,000+ Gst Rate 12 %

Product C 2,00,000 Gst Rate 18 %

Due date of Payment July-18 of Rs. 397,000/- of Total Invoice

But Actual date of Payment has Sep-18 With Interest Amt Rs. 12000/-

So Please tell me Whats Rate apply in Interest Amount

Thanks With Regards

Ravi SADhiyan