jQuery security and efficiency simple calculation code
Security:

If you want to check amounts securely, don't do it in the browser alone: you need a server-side validation (e.g. with PHP if it is your language of choice).

Efficiency:


You can use $('...') instead of jQuery('...') to improve the readability and concision (source: jquery.com).
Avoid looking for divs with jQuery every time you need them by assigning them to a variable. In this case, caching $('#total-amount') seems unnecessary, but it will pay off as soon as you call it once more (see jQuery Best Practices for more information).


Thus:

//AMOUNT CALCULATOR
var totalAmount = serviceAmount = 0;
// caching $('#total-amount')
var totalAmountDiv = $('#total-amount');

$('.select-service').click(function(){
    // caching $(this)
    var thisDiv = $(this);
    var serviceAmount = parseInt(thisDiv.parent().html().replace(/\D/g,''));
    if(thisDiv.is('.selected')){
        thisDiv.removeClass('selected');
        totalAmount -= serviceAmount;
        totalAmountDiv.html(totalAmount);
    }else{
        thisDiv.addClass('selected');
        totalAmount += serviceAmount;
        totalAmountDiv.fadeIn('slow').html(totalAmount);
    }
});
//AMOUNT CALCULATOR