JSFiddle - React, Tailwind, and code Playground

HTML

Your order total: <input type="text" id="totalbox" name="totalbox" size="10" maxlength="10" readonly="readonly" value="0.00" /><br>

<input name="Order" type="submit" value="Order" onclick ="JavaScript: calculateTotal();">

JavaScript

function calculateTotal()
{
    var total = "5.00";
    var totalbox = document.getElementById('totalbox');
	totalbox.value = "£" + total;
}