How to change total product weight?

[taitai]

I want to change total product weight "0" if order total quantity is larger than 1 in shopping cart.
When the customer order multiple items, I want to show each shipping method and shipping is $0 in checkout.

For example...
Express Mail $0
Priority Mail $0
UPS $0

This is for calculate the shipping after the order was completed. But I want to remain that the customer can select the shipping method.

Which php file should I change and what kind of code do I add ?

[taitai]

I solved it.

I just added

if ($_SESSION['cart']->count_contents() >1 ) {
$shipping_weight =0;
}