Apps Script code to automatically replace template variables in your invoice document.

automatic_invoice.gs

///////////////////////////////////////////////////////////////////////////////////////////////
// BEGIN EDITS ////////////////////////////////////////////////////////////////////////////////

const TEMPLATE_FILE_ID = 'YOUR_TEMPLATE_FILE_ID_HERE';
const DESTINATION_FOLDER_ID = 'YOUR_DESTINATION_FOLDER_ID_HERE';
const CURRENCY_SIGN = '$';

// END EDITS //////////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////////////////////
// WARNING: EDITING ANYTHING BELOW THIS LINE WILL CHANGE THE BEHAVIOR OF THE SCRIPT. //////////
// DO SO AT YOUR OWN RISK.//// ////////////////////////////////////////////////////////////////
// ----------------------------------------------------------------------------------------- //

// Converts a float to a string value in the desired currency format
function toCurrency(num) {
    var fmt = Number(num).toFixed(2);
    return `${CURRENCY_SIGN}${fmt}`;
}

// Format datetimes to: YYYY-MM-DD
function toDateFmt(dt_string) {
  var millis = Date.parse(dt_string);
  var date = new Date(millis);
  var year = date.getFullYear();
  var month = ("0" + (date.getMonth() + 1)).slice(-2);
  var day = ("0" + date.getDate()).slice(-2);

  // Return the date in YYYY-mm-dd format
  return `${year}-${month}-${day}`;
}


// Parse and extract the data submitted through the form.
function parseFormData(values, header) {
    // Set temporary variables to hold prices and data.
    var subtotal = 0;
    var discount = 0;
    var response_data = {};

    // Iterate through all of our response data and add the keys (headers)
    // and values (data) to the response dictionary object.
    for (var i = 0; i < values.length; i++) {
      // Extract the key and value
      var key = header[i];
      var value = values[i];

      // If we have a price, add it to the running subtotal and format it to the
      // desired currency.
      if (key.toLowerCase().includes("price")) {
        subtotal += value;
        value = toCurrency(value);

      // If there is a discount, track it so we can adjust the total later and
      // format it to the desired currency.
      } else if (key.toLowerCase().includes("discount")) {
        discount += value;
        value = toCurrency(value);
      
      // Format dates
      } else if (key.toLowerCase().includes("date")) {
        value = toDateFmt(value);
      }

      // Add the key/value data pair to the response dictionary.
      response_data[key] = value;
    }

    // Once all data is added, we'll adjust the subtotal and total
    response_data["sub_total"] = toCurrency(subtotal);
    response_data["total"] = toCurrency(subtotal - discount);

    return response_data;
}

// Helper function to inject data into the template
function populateTemplate(document, response_data) {

    // Get the document header and body (which contains the text we'll be replacing).
    var document_header = document.getHeader();
    var document_body = document.getBody();

    // Replace variables in the header
    for (var key in response_data) {
      var match_text = `{{${key}}}`;
      var value = response_data[key];

      // Replace our template with the final values
      document_header.replaceText(match_text, value);
      document_body.replaceText(match_text, value);
    }

}


// Function to populate the template form
function createDocFromForm() {

  // Get active sheet and tab of our response data spreadsheet.
  var sheet = SpreadsheetApp.getActiveSheet();
  var last_row = sheet.getLastRow() - 1;

  // Get the data from the spreadsheet.
  var range = sheet.getDataRange();
 
  // Identify the most recent entry and save the data in a variable.
  var data = range.getValues()[last_row];
  
  // Extract the headers of the response data to automate string replacement in our template.
  var headers = range.getValues()[0];

  // Parse the form data.
  var response_data = parseFormData(data, headers);

  // Retreive the template file and destination folder.
  var template_file = DriveApp.getFileById(TEMPLATE_FILE_ID);
  var target_folder = DriveApp.getFolderById(DESTINATION_FOLDER_ID);

  // Copy the template file so we can populate it with our data.
  // The name of the file will be the company name and the invoice number in the format: DATE_COMPANY_NUMBER
  var filename = `${response_data["Invoice Date"]}_${response_data["Company Name"]}_${response_data["Invoice Number"]}`;
  var document_copy = template_file.makeCopy(filename, target_folder);

  // Open the copy.
  var document = DocumentApp.openById(document_copy.getId());

  // Populate the template with our form responses and save the file.
  populateTemplate(document, response_data);
  document.saveAndClose();
    
}

@jimzdat Were you able to figure out the concatenating issue?

I did have it working correctly, but honestly don't recall what I had to do.
That was an initial install, and when I did a future install for another application, it was working fine - so not sure if something got changed in the code?