JSFiddle - React, Tailwind, and code Playground

HTML

<table><tr>
  <td>Value:</td>
  <td>
    <input name="value" type="text" id="value" />
  </td>
</tr>

<tr>
  <td>Postage:</td>
  <td>
    <input id="express" type="radio"    name="postageradio" value="3.50" />
    Express post ($3.50)

    <input id="registered" type="radio" name="postageradio" value="5.00" />
    Registered post ($5.00)
  </td>
</tr>

<tr>
  <td>Total:</td>
  <td colspan="2">
    <input name="total" type="text" readonly="readonly" id="total" />
  </td>
</tr>
</table>

JavaScript

function calctotal() {
  var value = $('#value').val();
  var amount = parseFloat(value) || 0;
  var post = parseFloat($("input[name=postageradio]:checked").val()) || 0;
    
  $('#total').val('$' + (amount + post).toFixed(2));
}

$(document).ready(function() {
  $('#value').keyup(function(event) {
    calctotal();
  } );

  $('#express, #registered').change(function(event) {
    calctotal();
  } );
} );