You're trying to do too many things in too many routes. just create a route called something like process-order then you could have an async controller of that route which would contain separate functions for

1. Storing the order details in mongodb.
2. Create an invoice
3. Send a token that allows access to download page.

first , just await the function to complete then call the next.

exports.processOrder = async (req, res, next) => {
try {
  const newOrder = await Order.create({
    order: req.params.id,
    details: req.body.details
  })
  
  const createInvoice = () => {
    return newOrder._id;
  }
  const invoice = createInvoice();
  
  const token = jwt.sign(
    {
       invoiceId = newOrder._id,
    },
    process.env.JWT_SECRET,
    {
      expiresIn: '24h'
    }
   );
    return res.status(200).json({status: 'success', message: 'you have 24 hours to download your digital goods', token})
  
} catch (error) {
  res.status(500).send(error)
}
}

This is a very basic idea, but basically store all of your info in one controller, then send a token with the id of the invoice, then when they go to download the book you would have a route where you verify the token. If it succeeds then the book is sent. like this:

app.use(
  '/book-download',
  expressJwt({ secret: process.env.JWT_SECRET, algorithms: ['HS256'] })
);

app.get('/book-download/success', async (req, res) => {
  try{
    const invoiceId = req.user.invoiceId;
    const invoice = await Invoice.find({_id: invoiceId})
    if (invoice) {
      return res.status(200).json({status: 'success', message: 'congratulations on your new download', data: {E-book})
    } else {
      return.res.status(404).json({status: 'fail', message: 'could not find an invoice with that ID'})
  } catch (error) {
    return res.send(error)
    }
});

You can choose to send the pdf via express, or you can allow them to enter a certain part of the website if the return is valid. There you go, that's an idea.

In Express check out, you need to define success / cancel handler

Ref here : https://www.section.io/engineering-education/nodejs-paypal-checkout/

In the success handler, based on the returned URI => you will get a Payer ID and payment ID. Based on that retrieve the transactio information (that internally contains yours order ID). Update your Order id as scucess / cancelled based on the paypal response.

app.get('/success', (req, res) => {
  const payerId = req.query.PayerID;
  const paymentId = req.query.paymentId;

  const execute_payment_json = {
    "payer_id": payerId,
    "transactions": [{
        "amount": {
            "currency": "USD",
            "total": "25.00"
        }
    }]
  };

// Obtains the transaction details from paypal
  paypal.payment.execute(paymentId, execute_payment_json, function (error, payment) {
      //When error occurs when due to non-existent transaction, throw an error else log the transaction details in the console then send a Success string reposponse to the user.
    if (error) {
        console.log(error.response);
        throw error;
    } else {
        console.log(JSON.stringify(payment));
        res.send('Success');
    }
});
});