GST Rate on Interest Invoice

Mr. A sold goods to Mr. B and against which payment is made through by opening Letter of Credit (LC) by Mr. B for 90 Days.

In the above scenario Mr. A will receive payment but Bank will charge the interest on that payment amount for 90 Days. So Mr. A will have to pay interest on payment amount for 90 Days. But this interest is to be born by Mr. B as per agreement between Mr. A and Mr. B.

Now, Mr. A will issue invoice of Interest amount to Mr. B. What will be GST Rate charge by Mr. A in the Invoice? Gst rate applicable to Goods (Say 5%) or 18%.