Free Shipping over set Amount with Marketpress?

A client asked me if I could set Marketpress to offer free shipping over a set amount. I saw the thread: where nickd32 says to add the following into a plugin.

$threshold = 100; //Free shipping for orders over 100 dollars
//final shipping amount is saved in a variable called $price
if ($total >= $threshold) { $price = 0; }

Does Marketpress have the capability to do this now without adding this? If not, what else would I need to put in the plugin file to get it work correctly...I am new to writing plugins. I can figure out how to get it to display as a plugin, but how will it know to hook to Marketpress?