Add option for add-on item to WooCommerce cart - A simple way to allow customers add gift wrap or other small add-on to their order. https://www.damiencarbery.com/2020/06/add-option-for-add-on-item-to-woocommerce-cart/

add-gift-wrap-option-to-woocommerce-cart.php

<?php
/*
Plugin Name: Add option for add-on item to WooCommerce cart
Plugin URI: https://www.damiencarbery.com/2020/06/add-option-for-add-on-item-to-woocommerce-cart/
Description: A simple way to allow customers add gift wrap or other small add-on to their order.
Author: Damien Carbery
Version: 0.3
*/


// Return product ID of the optional product.
function get_optional_product_id() {
	return 7923;
}

// Add option to add Gift Tag/Ribbon to an order.
// Inspired by http://wordpress.stackexchange.com/a/128646/57684
add_action('woocommerce_cart_totals_after_shipping', 'dcwd_gift_wrap_option_after_cart');
//add_action('woocommerce_review_order_before_cart_contents', 'dcwd_gift_wrap_option_after_cart');  // Show in checkout.
function dcwd_gift_wrap_option_after_cart() {
	// If you want to disable the gift wrap option when the cart
	// subtotal is below a certain amount then uncomment the next 3 lines.
	//if ( WC()->cart->subtotal_ex_tax < 40 ) {
	//	return;
	//}


	$enable_popup = true;  // Whether to create a popup with a photo and description of the gift wrap option.
    
	// If this code is used in the checkout then prettyPhoto will not work because the 'Your Order' section
	// is reloaded with ajax and the prettyPhoto init code won't find the prettyPhoto link.
	if ( is_checkout() ) {
		add_action( 'wp_footer', 'dcwd_trigger_prettyphoto_in_checkout' );
	}

	$found = false;

	$product_cart_id = WC()->cart->generate_cart_id( get_optional_product_id() );
	// Returns an empty string, if the cart item is not found
	$found = WC()->cart->find_product_in_cart( $product_cart_id );
	
	// Initialise these in case the product cannot be accessed.
	$product_name = '';
	$product_short_description = '';  
	$product_full_description = '';

	$gift_product = wc_get_product( get_optional_product_id() );
	
	if ( $gift_product ) {
		$product_name = $gift_product->get_name();
		$product_image = wp_get_attachment_image_url( $gift_product->get_image_id(), 'woocommerce_single' );
		$product_short_description = get_the_excerpt( $gift_product->get_id() );
		$product_full_description = apply_filters( 'the_content', get_the_content( null, false, $gift_product->get_id() ) );
		$ribbon_product_price = $gift_product->get_price_html();
	}
	else {
		$product_image = esc_url( wc_placeholder_img_src( 'woocommerce_single' ) );
		$ribbon_product_price = '&euro;1';
	}

	if ( $enable_popup ) {
		// Use prettyPhoto lightbox that is part of WooCommerce (though deprecated).
		wp_enqueue_script( 'prettyPhoto' );
		wp_enqueue_script( 'prettyPhoto-init' );
		wp_enqueue_style( 'woocommerce_prettyPhoto_css' );        
		// Or use the very old Thickbox that is part of WordPress.
		//add_thickbox();
		
		$gift_wrap_markup = sprintf( '<a id="gift-tag-popup thickbox" data-rel="prettyPhoto" href="%s" title="%s">%s <span class="dashicons dashicons-info"></span></a>',
							$product_image, $product_full_description, $product_name );
	}
	else {
		$gift_wrap_markup = $product_name;
	}
?>
    <tr class="ribbon">
        <th><?php echo $gift_wrap_markup; ?></th>
        <td>
<?php
    // If product not found, add it.
    if ( ! $found ) {
?>
        <a href="<?php echo do_shortcode(sprintf('[add_to_cart_url id="%d"]', get_optional_product_id())); ?>"><?php echo $product_short_description; ?></a> (+<?php echo $ribbon_product_price; ?>)
    </tr>
<?php } else { ?>
        Yes
<?php }
?></td>
    </tr>
<?php
}


// Make 'Order notes' a required field if the gift wrap product is in the cart.
add_filter( 'woocommerce_checkout_fields', 'dcwd_make_order_notes_required' );
function dcwd_make_order_notes_required( $fields ) {
	if ( array_key_exists( 'order', $fields ) ) {
		$product_cart_id = WC()->cart->generate_cart_id( get_optional_product_id() );
		if ( WC()->cart->find_product_in_cart( $product_cart_id ) ) {
			$fields[ 'order' ][ 'order_comments' ][ 'required' ] = true;
		}
	}
	
	return $fields;
}


// Add a message on checkout page to remind customers with the gift wrap to include info in the Order Notes field.
add_action( 'woocommerce_before_checkout_form', 'dcwd_order_notes_reminder' );
function dcwd_order_notes_reminder() {
	$product_cart_id = WC()->cart->generate_cart_id( get_optional_product_id() );
	if ( WC()->cart->find_product_in_cart( $product_cart_id ) ) {
?>
<script>
jQuery(function( $ ){
	if ( '' == $( '#order_comments' ).val() ) {
		alert( "Don't forget to add your Gift Tag & Ribbon message to the 'Order notes' section." );
	}
});
</script>
<?php
	}
}


// If this code is used in the checkout then prettyPhoto will not work because the 'Your Order' section
// is reloaded with ajax and the prettyPhoto init code won't find the prettyPhoto link.
function dcwd_trigger_prettyphoto_in_checkout() {
?>
<script>
jQuery('body').on('updated_checkout', function(){
    jQuery("a.zoom, a[data-rel^='prettyPhoto']").prettyPhoto({
		hook: 'data-rel',
		social_tools: false,
		theme: 'pp_woocommerce',
		horizontal_padding: 20,
		opacity: 0.8,
		deeplinking: false
	});
});
</script>
<?php
}