OK, solved that (in fact I've solved almost all my questions i've asked on forum myself :) )
So, just in case someone comes here for the same task , below is the code for the modifier (as usuall - you have to enable it in _config.php, build DB , and then tweak some SS files to ensure it shows up on receipts and reports)
<?php
class PriceShippingModifier extends OrderModifier {
function LiveAmount() {
$order = $this->Order();
$total = $order->SubTotal();
//If total is more than 200 than delivery is free, otherwise - charge 5 bux
if ($total<=200){ return 5; }
else { return 0; }
}
function CartTitle() {
$order = $this->Order();
$total = $order->SubTotal();
if ($total<=200){ return "Delivery"; }
else { return "Free Delivery"; }
}
function TableTitle() {
$order = $this->Order();
$total = $order->SubTotal();
if ($total<=200){ return "Delivery"; }
else { return "Free delivery"; }
}
}