I don't understand what you're trying to do with this part.
<select name="<?php "Quantity[$ID]\" id=\"Quantity.$ID\" "?>"> <option value="<?php $row['Quantity']; ?>">"<?php $row['Quantity']; ?>"</option>
</select>
For one thing, this is not a valid statement:
<?php "Quantity[$ID]\" id=\"Quantity.$ID\" "?>
Nor is this (twice):
<?php $row['Quantity']; ?>
It might help you if you start by writing what you're trying to accomplish in simple HTML, then go back and write it using PHP. If you can't write a valid HTML form you won't be able to write it using PHP.