Has two towels

function bbloomer_add_gift_if_sku_added_cart( $passed, $product_id, $quantity ) {
global $woocommerce;


$skuswithgift = array('SMWB-M23','001-SLW');

$giftsku = 'comb';

$coupon_code = 'combfree';

$product = wc_get_product( $product_id );

$total_towels = 0;

// Determine how many towels there are
// Loop through every cart item
foreach ( $woocommerce->cart->get_cart() as $cart_item ) {

  // $product = $cart_item['data'];
  $product = $cart_item;
  
  // Is it in Gift SKUs
  $is_towel = in_array($product->get_sku(), $skuswithgift);
  if($is_towel){
    // Add this quantity to the total towels
    $total_towels += $cart_item['quantity'];
  }
  
}


// Apply Discount
if ($total_towels < 1) {
  WC()->cart->add_to_cart( wc_get_product_id_by_sku($giftsku) );
  wc_add_notice( __( 'Hey there! As promised, you recieved a free comb with the purchase of two towels and we added it to your cart for you!', 'woocommerce' ), 'success' );
  $woocommerce->cart->add_discount( $coupon_code );
}

else {
  WC()->cart->remove_cart_item( wc_get_product_id_by_sku($giftsku) );
  $woocommerce->cart->remove_coupon( $coupon_code );
}

return $passed;
}

add_filter( 'woocommerce_add_to_cart_validation', 'bbloomer_add_gift_if_sku_added_cart', 10, 3 );