You can see from the title that this is not done how you should
do it. However some people have no idea how to work with XML.
Also, this way works just as well. And it’s easier and faster.
You copy the form.phtml to your theme. On the image blow you
see it done in the Ultimo Magento theme from Infortis.
In it I added;
<div id="price-incl-tax"><?php echo('Alle
prijzen zijn inclusief BTW') ?></div>
And than in my own custom css file I added the design code;