If anyone is still maintaining or updating any version of this, then perhaps they could include this fix.

I use table prefixes, and part of the code in /includes/functions/functions_taxes.php is not compatible, this is what you need to do to fix it.

Before: @line ~49

$customers_default_address_id = $customer_check->fields['customers_default_address_id'];
$address_book_check = $db->Execute("select * from address_book where address_book_id = $customers_default_address_id and customers_id = $customers_id");
$entry_tva_intracom = $address_book_check->fields['entry_tva_intracom'];

After:

$customers_default_address_id = $customer_check->fields['customers_default_address_id'];
$address_book_check = $db->Execute("select * from ".TABLE_ADDRESS_BOOK." where address_book_id = $customers_default_address_id and customers_id = $customers_id");
$entry_tva_intracom = $address_book_check->fields['entry_tva_intracom'];


Notice I have just added the define reference to TABLE_ADDRESS_BOOK.

Another problem I have had was with the install SQL statements, I am not sure why but they don't seem to be able to do the SELECT from the configuration group table in order to get the configuration_group_id. I just modified the statements to manually put the configuration group id in.

I am installing on Zen cart 1.5.1 and apart from these problems it seems to work fine so far - but I have not yet gotten to testing it fully.