JSFiddle - React, Tailwind, and code Playground

HTML

<table id="tabela">


  <tfoot>
    <tr>

      <td colspan="3"></td>
      <td align="right"> <span id="total">Total: <span class="total">0</span></span>
        <p><span class="aviso">Some se o item A estiver selecionado</span></p>
        <p><span class="aviso">Some se o item B estiver selecionado</span></p>
        <p><span class="aviso">Some se o item C estiver selecionado</span></p>
        <p>Soma só as unidades(select): <span id="total_unidades"></span></p>
      </td>
    </tr>
  </tfoot>

  <tbody>
    <tr>
      <td>
        <input type="checkbox" value="16" class="price" id="chk1"> item A (16) X</td>
      <td id="e1">
        <select value="" class="qty" name="qty">
          <option value="1">1 unidade</option>
          <option value="2">2 unidades</option>
          <option value="3">3 unidades</option>
          <option value="4">4 unidades</option>
        </select>
        <span id="amount" class="amount">0</span>
      </td>

    </tr>

    <tr>
      <td>
        <input type="checkbox" value="45" class="price" id="chk2"> item B (45) X</td>
      <td id="e2">
        <select value="" class="qty" name="qty">
          <option value="1">1 unidade</option>
          <option value="2">2 unidades</option>
          <option value="3">3 unidades</option>
          <option value="4">4 unidades</option>
        </select>
        <span id="amount" class="amount">0</span>
      </td>

    </tr>

    <tr>
      <td>
        <input type="checkbox" value="61" class="price" id="chk3"> item C (61) X</td>
      <td id="e3">
        <select value="" class="qty" name="qty">
          <option value="1">1 unidade</option>
          <option value="2">2 unidades</option>
          <option value="3">3 unidades</option>
          <option value="4">4 unidades</option>
        </select>
        <span id="amount" class="amount">0</span>
      </td>

    </tr>


  </tbody>
</table>

JavaScript

$(document).ready(function() {
  update_amounts();
  $('.qty, .price').change(update_amounts);
});


function update_amounts() {
  var sum2 = 0;
  var sum = 0;
  $('select').hide();
  $('tr').each(function() {
    var unidades = $(this).find('option:selected');
      var checkbox = $(this).find('.price');
    if (checkbox.is(':checked')) {
      sum2 += parseInt(unidades.val(), 10);
    }
  });
  $('#total_unidades').text(sum2);


  $('table input').each(function(i) {
    var tr = $(this).closest('tr');
    var select = tr.find('.qty, .amount').toggle(this.checked);
    var qty = select.val();
    var price = this.checked ? this.value : 0;
    var amount = (qty * price) || 0;
    if (!this.checked) select.prop('selectedIndex', 0);
    sum += amount;
    tr.find('.amount').text(amount);
    $('table').find('tfoot .aviso').eq(i).toggle(!amount);
    
  });
  $('#total').toggle(!!sum);
  $('.total').text(sum);
}