[stuff]

var Billing = Class.create();
Billing.prototype = {
    initialize: function(form, addressUrl, saveUrl){
        this.form = form;
        if ($(this.form)) {
            $(this.form).observe('submit', function(event){this.save();Event.stop(event);}.bind(this));
        }
        this.addressUrl = addressUrl;
        this.saveUrl = saveUrl;
        this.onAddressLoad = this.fillForm.bindAsEventListener(this);
        this.onSave = this.nextStep.bindAsEventListener(this);
        this.onComplete = this.resetLoadWaiting.bindAsEventListener(this);
    },

    setAddress: function(addressId){
        if (addressId) {
            request = new Ajax.Request(
                this.addressUrl+addressId,
                {method:'get', onSuccess: this.onAddressLoad, onFailure: checkout.ajaxFailure.bind(checkout)}
            );
        }
        else {
            this.fillForm(false);
        }
    },

    newAddress: function(isNew){
        if (isNew) {
            this.resetSelectedAddress();
            Element.show('billing-new-address-form');
        } else {
            Element.hide('billing-new-address-form');
        }
    },

    resetSelectedAddress: function(){
        var selectElement = $('billing-address-select')
        if (selectElement) {
            selectElement.value='';
        }
    },

    fillForm: function(transport){
        var elementValues = {};
        if (transport && transport.responseText){
            try{
                elementValues = eval('(' + transport.responseText + ')');
            }
            catch (e) {
                elementValues = {};
            }
        }
        else{
            this.resetSelectedAddress();
        }
        arrElements = Form.getElements(this.form);
        for (var elemIndex in arrElements) {
            if (arrElements[elemIndex].id) {
                var fieldName = arrElements[elemIndex].id.replace(/^billing:/, '');
                arrElements[elemIndex].value = elementValues[fieldName] ? elementValues[fieldName] : '';
                if (fieldName == 'country_id' && billingForm){
                    billingForm.elementChildLoad(arrElements[elemIndex]);
                }
            }
        }
    },

    setUseForShipping: function(flag) {
        $('shipping:same_as_billing').checked = flag;
    },

    save: function(){
        if (checkout.loadWaiting!=false) return;

        var validator = new Validation(this.form);
        if (validator.validate()) {
            checkout.setLoadWaiting('billing');

            var request = new Ajax.Request(
                this.saveUrl,
                {
                    method: 'post',
                    onComplete: this.onComplete,
                    onSuccess: this.onSave,
                    onFailure: checkout.ajaxFailure.bind(checkout),
                    parameters: Form.serialize(this.form)
                }
            );
        }
    },

    resetLoadWaiting: function(transport){
        checkout.setLoadWaiting(false);
    },

    nextStep: function(transport){
        if (transport && transport.responseText){
            try{
                response = eval('(' + transport.responseText + ')');
            }
            catch (e) {
                response = {};
            }
        }

        if (response.error){
            if ((typeof response.message) == 'string') {
                //alert(response.message);
            } else {
                if (window.billingRegionUpdater) {
                    billingRegionUpdater.update();
                }
                //alert(response.message.join("\n"));
            }

            return false;
        }

        checkout.setStepResponse(response);
        payment.initWhatIsCvvListeners();
    }
}

в данном случае вызывается billing.save()
остальные классы с аналогичными функциями