This works:
var form = $(this).closest('form');
form = form.serializeArray();
form = form.concat([
{name: "customer_id", value: window.username},
{name: "post_action", value: "Update Information"}
]);
$.post('/change-user-details', form, function(d) {
if (d.error) {
alert("There was a problem updating your user details")
}
});