Hi,
Normally this "return" url will be taken care by the hidden inputs:
such as;
<input type="hidden" name="option" value="com_virtuemart" />
<input type="hidden" name="view" value="cart" />
<input type="hidden" name="task" value="setpayment" />
<input type="hidden" name="controller" value="cart" />
So the once the form is submitted, it goes back to shopping cart page.
But forcing the <form> element to return back to "cart" page by including the CART url in the action="" attribute, is just an another way of doing it. It won't hurt. So you did it correctly.
Alternatively, you can do it something like;
<?php echo JRoute::_('index.php/cart/'); ?>
or
<?php echo JRoute::_('index.php?option=com_virtuemart&view=cart'); ?>
They are all the same.
Regards.
Support.