<div id="total_sum">
  <!-- here will show up total amount, which is sum of unit[] * price[] - discount[] of all below form-rows -->
</div>

<div id="total_discount">
  <!-- here will show up total discount amount, which is sum of discount[] of all below form-rows -->
</div>

<div id="total_net">
  <!-- here will show up net amount, which is total_sum - total_discount -->
</div>

<form method="POST" action="">

  <div class="form-row">
    <select name="item[]">
      <option value="item-id-1">Item 1</option>
      <option value="item-id-2">Item 2</option>
      <option value="item-id-3">Item 3</option>
    </select>
    <input type="number" name="unit[]" />
    <input type="number" name="price[]" />
    <input type="number" name="discount[]" />
    <input type="number" name="sum[]" />
  </div>

  <div class="form-row">
    <select name="item[]">
      <option value="item-id-1">Item 1</option>
      <option value="item-id-2">Item 2</option>
      <option value="item-id-3">Item 3</option>
    </select>
    <input type="number" name="unit[]" />
    <input type="number" name="price[]" />
    <input type="number" name="discount[]" />
    <input type="number" name="sum[]" />
  </div>

  <!-- and users can dynamically add as many form-rows as they want -->

  <div class="form-row">
    <select name="item[]">
      <option value="item-id-1">Item 1</option>
      <option value="item-id-2">Item 2</option>
      <option value="item-id-3">Item 3</option>
    </select>
    <input type="number" name="unit[]" />
    <input type="number" name="price[]" />
    <input type="number" name="discount[]" />
    <input type="number" name="sum[]" />
  </div>

</form>