User.signup(username, password, address, paymentPlan).then(function(user) {
  user.sendConfirmationEmail();
});

$.ajax({
  url: "meaninglessurltoafrontenddeveloper",
  method: "POST",
  data: {
    username: username,
    password: password,
    address: address,
    paymentPlan: paymentPlan
  }
  success: function(response) {
    var parsedUser = parseUser(response);
    parsedUser.sendConfirmationEmail();
  },
  error: function(response) {
    alert("Oops");
  }
});