If your customers are getting an order confirmation email that doesn't have a tax line, but you want the tax to be displayed in the email, try this fix in your PrestaShop 1.5x store.
First, edit the file: /classes/PaymentModule.php
Around line 558, find the line:
$invoice_state = $invoice->id_state ? new State($invoice->id_state) : false;
Immediately following this line, insert the following two lines:
$total_paid_notax = $order->total_products; + $order->total_shipping + $order->total_wrapping - $order->total_discounts;
$total_tax = $order->total_paid - $total_paid_notax - $order->total_shipping;
Then, just after line 600, find the following lines:
'{total_discounts}' => Tools::displayPrice($order->total_discounts, $this->context->currency, false),
'{total_shipping}' => Tools::displayPrice($order->total_shipping, $this->context->currency, false),
'{total_wrapping}' => Tools::displayPrice($order->total_wrapping, $this->context->currency, false));
Change the last line above to:
'{total_wrapping}' => Tools::displayPrice($order->total_wrapping, $this->context->currency, false),
And add the following line directly below the above line:
'{total_tax}' => Tools::displayPrice($total_tax, $currency, false));
Save the file.
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Next, edit the file: /mails/en/order_conf.html
About line 55, find the following code:
<tr style="text-align: right;">
<td> </td>
<td style="background-color: #ebecee; padding: 0.6em 0.4em;" colspan="3">Gift-wrapping</td>
<td style="background-color: #ebecee; padding: 0.6em 0.4em;">{total_wrapping}</td>
</tr>
And directly below that insert the following code:
<tr style="text-align: right;">
<td> </td>
<td style="background-color: #ebecee; padding: 0.6em 0.4em;" colspan="3">Tax</td>
<td style="background-color: #ebecee; padding: 0.6em 0.4em;">{total_tax}</td>
</tr>
Save the file.
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In most cases, this should show a separate tax line in the customer's order confirmation email. Hope this helps...