SampleJS

function calculateBilling() { 
	let basicServiceCost = 20; // R20 per month   //consider the difference var / let
	let premiumServiceCost = 30; // R30 per month
	let CALLS = 100;	
	let MINUTES = 500;
	let totalBill = 0; 
	const customerID = document.getElementById('customerID').value; 
	const callsMade = parseInt(document.getElementById('callsMade').value); 
	const callMinutes = parseInt(document.getElementById('callMinutes').value); 
	
	if(callsMade > CALLS) { 
		totalBill = basicServiceCost + (callMinutes * 0.5); // R0.50 per minute for extra calls
	} 
	else { 
			totalBill = premiumServiceCost; 
	}
	let output = "Customer ID: " + customerID + " Billing Total: R" + totalBill.toFixed(2);
	document.getElementById('billingResult').innerHTML = output;
}