This is an example of what we normaly do in OOP

Invoice.js

class Invoice {
    ...
    doCheckout(shippingDetails){
        if(this.validate(shippingDetails)){
            this.shippingDetails = shippingDetails
            var serializedInstance = JSON.stringify(this)
            $OurAjaxService.post({
                url: this.url,
                data: serializedInstance
            }).then((response) => {
                //...notify the user or something, trigger the state
            })
        }
    }
    ...
}