Change variable with radio buttons

Hallo !
I’m comming back with a very last question about changing a variable depending on check radio button, and it’s a PHP oriented issue.

I’m using a basic function to calculate $postage price. This postage depends on the $total. But instead of this, I would to be able to calculate $postage with three differents choices : France/Europe/Monde, three radio buttons.

This is my actual function

function cart_postage($total) {
	    switch ($total) {
	        case ($total < 2): // entre 0 et 2
	            $postage = 5;
	        case ($total < 5): // entre 2 et 5
	            $postage = 10;
	        case ($total < 10): // entre 5 et 10
	            $postage = 5.5;
            case ($total < 30): // entre 10 et 30
	            $postage = 10;
	            $postage = 1;
	    return $postage;

and my buttons

        <label class="france"><input type="radio" id='france' value="10" value="France"/>France</label>
        <label class="europe"><input type="radio" id='europe' value="15" value="Europe"/>Europe</label>
        <label class="monde"><input type="radio" id='monde' value="20" value="Monde"/>Monde</label>

The idea is : if I click on “Europe”, the $postage price is refreshing to its value : 15.
I already tried something with param() and isset but it was a total disaster.

If you have some times to help me, or some advices, it could be great !