flow.js

async.waterfall([
  soap.createClient.bind(null, process.cwd() + '/util/usaepay.wsdl'),
  function(client, cb) {
    client.runSale(args, cb);
  },
  function(result, body, cb) {
    if (result.runSaleReturn.ResultCode.$value === 'E') {
      return new Error('failed to process');
    } else if (result.runSaleReturn.ResultCode.$value === 'A') {
      return cb(null, result);
    } else {
      return cb('declined');
    }
  function(result, cb) {
    app.get('db').verifyAd(result.runSaleReturn.RefNum.$value, req.session.user.id, cb);
  },
  function(result, cb) {
    sendMail(req.session.user.email,
             app.get('transporter'),
             details.service_type,
             mixin(result, { price : details.price }),
             cb);
  }
], function(err, result) {
  if (err === 'declined') {
    res.render('declined');
  } else if (err) {
    next(err);
  } else {
    res.render('authorized');
  }
});

With promises this would look like (I'm assuming here):

soap.clientClient(process.cwd() + '/util/usaepay.wsdl').chain(function(client) {
  return client.runSale(args);
}).chain(function(result) {
  if (result.runSaleReturn.ResultCode.$value === 'E') {
    return Future.rejected(err);
  } else if (result.runSaleReturn.ResultCode.$value === 'A') {
    return Future.of(result);
  } else {
    return Future.rejected('declined');
  }
}).chain(function(result) {
  return db.verifyAd(result.runSaleReturn.RefNum.$value, req.session.user.id);
}).chain(function(result) {
  return sendMail(req.session.user.email,
                  transporter,
                  details.service_type,
                  mixin(result, {price: details.price}));
}).fork(function(err) {
  if (err === 'declined') {
    res.render('declined');
  } else {
    next(err);
  }
}, function() {
  res.render('authorized');
});