It can be done easily with a custom hooked function displaying an additional button that will add 12 products on 1 click on single product pages for simple products only:
add_action( 'woocommerce_after_add_to_cart_button', 'additional_simple_add_to_cart', 20 );
function additional_simple_add_to_cart() {
global $product;
// Only for simple product type
if( ! $product->is_type('simple') ) return;
$href = '?add-to-cart=' . esc_attr( $product->get_id() ) . '&quantity=12';
$class = 'ingle_add_to_cart_button-12 button alt';
$style = 'display: inline-block; margin-top: 12px;';
$button_text = __( "Add a case of 12", "woocommerce" );
// Output
echo '<br><a rel="no-follow" href="'.$href.'" class="'.$class.'" style="'.$style.'">'.$button_text.'</a>';
}
Code goes in function.php file of your active child theme (active theme).
Tested and works.
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