well, i'm looking at the code right now, and it looks as if you could do something along these lines, but someone could maybe tell me if there is a better way to do this:
in checkout_shipping.php:
below
// get all available shipping quotes
$quotes = $shipping_modules->quote();
add some code like this:
if ($order->info['total'] > 100) //if the order total is more than 100$
{
add to the quotes array another option that is free shipping
(i have yet to figure out how the $quotes array is structured, so if anyone knows, that would be great help, if not, i'll have to sort through some more code)
}