Fading and Loader jQuery Improvement
Let's go through it one section at a time.

jQuery.fn.center = function () {
            this.css("position","absolute");
            this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");
            this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
            return this;
        }



You don't have to explicitly append 'px' to your numeric values. jQuery is smart enough to 
correctly format your CSS styles.
Since this is a jQuery plugin, you return this; at the end of the function. This is good, as
it preserves chaining.
If the .center function is only used for the loading divs, consider removing it and instead
styling the divs with plain CSS.

#loader {
    position: fixed;
    top: 50%;
    left: 50%;
    margin-top: -50px; /* half of the height */
    margin-left: -100px; /* half of the width */
}
...





$( ".buttonFinish" ).on('click', function(event) {
    $('body').fadeOut('slow');
    $('#loader').center();
    $('#loading').center();
    $('#loader').show();
    $('#loading').show();



As mentioned by @Flambino, this particular code (as presented in the question) will fade out the entire body of the HTML document, which includes everything, even the loader elements. Perhaps the code that you're actually using is slightly different (is the selector .body or #body instead?).
It's best practice to enclose JavaScript that uses external dependencies inside an IIFE.
By doing so, you can use the "safe" name of an external dependency while still being able to
alias is to a more convenient identifier locally. In addition, you can freely create variables inside the scope
of the IIFE without polluting the global namespace. Your example code is fairly short, but you likely have much
more code on the page, and it is a good habit to have regardless. Here's one way to do it:

(function($) {
    $( ".buttonFinish" ).on('click', function(event) {
        ...
    });
})(jQuery);

You can select both elements at the same time, just like in CSS: $('#loader, #loading').
You can chain the .show() and .center() together: $('...').show().center().




    //Use the above function as:
    var request = $.ajax({
        url:'{{ URL::route('saveCart') }}',
        type:'POST'
        });

        request.done(function(msg) {
            $('.order_number').val(msg);
              $('#summaryForm').submit();
              event.preventDefault();
        });
    return false;
});



Since you're not using the request variable for anything other than .done(),
you can skip declaring it and simply chain .done() after $.ajax():

$.ajax({
    ...
}).done(function(msg) {
    ...
});



As an overall note, the indentation of the code (as it is in the question) seems haphazard,
making the control flow more difficult to see than it could be. Making matching
braces have the same indentation (and, in general, maintaining a consistent style)
will help make the code more readable and understandable.