You can download your invoice depending on how you paid for the charges.
If you paid directly through the payment gateway
If you paid your Kite API charges directly using the payment gateway on Kite Connect (Razorpay), you can download your invoice from the payment history on the billing page.
If charges were debited from your Zerodha account
If your Kite API charges were debited from your Zerodha account, you can download your invoice from Console.