Thank you for this information, but I got it working without premium templates extension.
This is my code for the invoice table:
<?php
$templater = WPI()->templater();
$order = $templater->order;
// Get cart items
$items = $order->get_items();
?>
<table>
<thead>
<tr>
<th>SKU</th>
<th>Quantity</th>
<th>Litre</th>
<th>Name</th>
<th>Single price<br/>EUR</th>
<th>Total price<br/>EUR</th>
</tr>
</thead>
<tbody>
<?php
foreach ( $items as $item ) {
$product_name = $item->get_name();
$product_id = $item->get_product_id();
$product_variation_id = $item->get_variation_id();
$product = wc_get_product( $product_id );
$sku = $product->get_sku();
?>
<tr>
<td><?php if($sku){ echo $sku; } else { echo '-'; } ?></td>
<td><?php echo wp_kses_post( apply_filters( 'woocommerce_email_order_item_quantity', $item->get_quantity(), $item ) ); ?></td>
<td><?php echo get_field('liter', $product->get_id()) ?></td>
<td><?php echo $product->get_name(); ?></td>
<td><?php echo wc_price( $order->get_item_total( $item, true, true ), array( 'currency' => $order->get_order_currency() ) ); ?></td>
<td><?php echo wp_kses_post( $order->get_formatted_line_subtotal( $item ) ); ?></td>
</tr>
<?php } ?>
<?php
foreach ( $invoice->get_order_item_totals() as $key => $total ) {
$class = str_replace( '_', '-', $key );
?>
<tr>
<td> </td>
<td> </td>
<td><?php do_action( 'wpi_order_item_totals_left', $key, $invoice ); ?> </td>
<td><?php echo $total['label']; ?></td>
<td> </td>
<td><?php echo str_replace( ' ', '', $total['value'] ); ?></td>
</tr>
<?php } ?>
</tbody>
</table>