Question

How can you add 25 when you check that box and subtract 25 when you uncheck it on the number that is inside the span id=”preciototal” ?

Take a look here please.
Its part of a checkout form and the idea is it calculates the total price:

http://jsfiddle.net/9YGBu/2/

Answer 1

As an alternative answer which demonstrates a few more advanced jQuery concepts to you, consider the following javascript:

$(document).ready(function() {
    // cache anything reused in variables
    var $zonaenvio = $("#zonaenvio");
    var $gelypilas = $('#gelypilas');
    var $totalprecio = $('#totalprecio');
    var $totalSpan = $('#preciototal');
    // declare a function that does everything
    var updatePrice = function() {
        // get the value of the selected product, or 0 if nothing selected
        var productAmount = parseInt($zonaenvio.val() || '0');
        if (productAmount > 0) {
            // if something selected, calculate correct total and show it
            var correctTotal = productAmount + ($gelypilas.is(':checked') ? 25 : 0);
            $totalSpan.text(correctTotal);
            $totalprecio.show('slow');
        } else {
            // otherwise, if nothing selected, hide the total information
            $totalprecio.hide();
        }
    };
    // bind the function to the input events
    $zonaenvio.change(updatePrice).change(); // trigger once if needed
    $gelypilas.change(updatePrice);

});

Does that make sense? See here for a working jsFiddle example.