[TelaFirma]

Try this:

{if $subtotal lt 30}
{math equation="30-x" x=$subtotal assign="more" }
If you order just {$more} more you can get free shipping 
{/if}

I have not tested this, but it should work just fine.

Here is the page in the Smarty Docs that references math equations in templates:

http://smarty.php.net/manual/en/language.function.math.php