You may have the same issue I had.

The webserver wouldn't serve index.php as the default index file, so any call to javascript would return 404, which was the wrong file type since it was expecting javascript, and it would get blocked.

The solution was go to the .htaccess file in the root folder of your AbanteCart installation and add this line:
DirectoryIndex index.php

A few years ago I made a customization to my order_invoice.tpl which I used for book keeping purposes. At the bottom of the file I added the following:

<?php echo $order['date_added']; ?>#
<?php echo $order['payment_method']; ?>#
<?php echo $total['text']; ?>#
<?php echo $order['payment_address']; ?>#
<?php echo $order['customer_telephone']; ?>#
<?php echo $order['customer_email']; ?>!#
<?php foreach ($order['product'] as $product) { ?><?php echo $product['name']; ?><?php } ?>

This created a text line at the bottom of the invoice with the date, payment method, total price, customer address, phone, email, and what they ordered. It all worked fine.

I recently updated to a newer version of AbanteCart and the payment_method stopped working. I used the following line to see what options I had:

<?php echo '<pre>'; print_r($order); echo '</pre>'; ?>

Much to my dismay, I found that payment_method is no longer in the array order at all. I would like to know how I can reenable payment_method so that my bookkeeping isn't lacking that information.