a solution to implement an add-on product that only appears when a customer orders a minimum quantity of the main product. This uses WooCommerce hooks and doesn't require a plugin

wooCommerce-add-on-product-with-minimum Quantity.php

<?php

/**
 * Add conditional add-on product option when minimum quantity is met
 */
function add_conditional_addon_product() {
    global $product;
    
    // Only run for specific product IDs (adjust as needed)
    $target_product_ids = array(123); // Replace with your main product ID
    
    // Add-on product details
    $addon_product_id = 456; // Replace with your add-on product ID
    $min_quantity = 10; // Minimum quantity to show add-on
    $addon_price = 7; // Add-on price
    
    if (in_array($product->get_id(), $target_product_ids)) {
        ?>
        <div id="conditional-addon" style="display: none; margin-top: 20px; padding: 15px; background: #f5f5f5; border: 1px solid #ddd;">
            <h4>Special Offer</h4>
            <p>Since you're ordering <?php echo $min_quantity; ?> or more, you can add a storage box for just $<?php echo $addon_price; ?>!</p>
            <label>
                <input type="checkbox" name="add_addon_product" id="add_addon_product">
                Add Storage Box (+$<?php echo $addon_price; ?>)
            </label>
        </div>

        <script type="text/javascript">
            jQuery(document).ready(function($) {
                // Check quantity on page load and quantity change
                function checkQuantity() {
                    var quantity = parseInt($('input.qty').val()) || 1;
                    if (quantity >= <?php echo $min_quantity; ?>) {
                        $('#conditional-addon').slideDown();
                    } else {
                        $('#conditional-addon').slideUp();
                        $('#add_addon_product').prop('checked', false);
                    }
                }
                
                // Initial check
                checkQuantity();
                
                // Check when quantity changes
                $('input.qty').on('change', function() {
                    checkQuantity();
                });
                
                // Add click handler for the checkbox
                $('form.cart').on('submit', function(e) {
                    if ($('#add_addon_product').is(':checked')) {
                        // Create a hidden input to pass the add-on selection
                        $(this).append('<input type="hidden" name="add_addon_to_cart" value="1">');
                    }
                });
            });
        </script>
        <?php
    }
}
add_action('woocommerce_after_add_to_cart_button', 'add_conditional_addon_product');

/**
 * Add the add-on product to cart when checkbox is checked
 */
function add_addon_product_to_cart($cart_item_key, $product_id, $quantity, $variation_id, $variation, $cart_item_data) {
    if (!isset($_POST['add_addon_to_cart']) || !$_POST['add_addon_to_cart']) {
        return;
    }
    
    // Add-on product details (must match above)
    $addon_product_id = 456; // Replace with your add-on product ID
    $min_quantity = 10; // Minimum quantity to show add-on
    
    // Verify the main product quantity meets minimum
    if ($quantity >= $min_quantity) {
        // Check if add-on is already in cart
        $addon_in_cart = false;
        foreach (WC()->cart->get_cart() as $cart_item) {
            if ($cart_item['product_id'] == $addon_product_id && isset($cart_item['addon_for']) {
                $addon_in_cart = true;
                break;
            }
        }
        
        // Add add-on to cart if not already there
        if (!$addon_in_cart) {
            WC()->cart->add_to_cart($addon_product_id, 1, 0, array(), array('addon_for' => $product_id));
        }
    }
}
add_action('woocommerce_add_to_cart', 'add_addon_product_to_cart', 10, 6);

/**
 * Hide add-on product from shop and search
 */
function hide_addon_product($query) {
    if (!is_admin() && $query->is_main_query() && (is_shop() || is_product_category() || is_product_tag() || is_search())) {
        $addon_product_id = 456; // Replace with your add-on product ID
        $post__not_in = $query->get('post__not_in');
        $post__not_in = is_array($post__not_in) ? $post__not_in : array();
        $post__not_in[] = $addon_product_id;
        $query->set('post__not_in', $post__not_in);
    }
}
add_action('pre_get_posts', 'hide_addon_product');

/**
 * Remove add-on if main product is removed from cart
 */
function remove_addon_if_main_removed($cart_item_key, $cart) {
    $cart_item = $cart->cart_contents[$cart_item_key];
    
    // If removed item has addon_for, it's an add-on - we don't need to do anything
    if (isset($cart_item['addon_for'])) {
        return;
    }
    
    $addon_product_id = 456; // Replace with your add-on product ID
    
    // Check if removed item is a main product that might have an add-on
    foreach ($cart->cart_contents as $key => $item) {
        if (isset($item['addon_for']) && $item['addon_for'] == $cart_item['product_id'] && $item['product_id'] == $addon_product_id) {
            $cart->remove_cart_item($key);
            break;
        }
    }
}
add_action('woocommerce_remove_cart_item', 'remove_addon_if_main_removed', 10, 2);
/*
Why this code was written?

This code was written on request posted on this reddit post. https://www.reddit.com/r/woocommerce/s/4S2SuBBWZY

*/                
/*

Setup Instructions
Replace 123 with your main product ID (the product that needs minimum quantity)

Replace 456 with your add-on product ID (the storage box product)

Adjust $min_quantity and $addon_price as needed

The code will:

Show the add-on option only when quantity ≥ 10

Add the add-on to cart when checkbox is checked

Keep the add-on linked to the main product

Remove the add-on if the main product is removed

Hide the add-on product from shop/category pages

Alternative Plugin Solutions
If you prefer a plugin approach, consider these options:

WooCommerce Product Add-ons - Official WooCommerce extension that allows conditional fields

Conditional Discounts for WooCommerce - Can set rules for add-ons based on quantity

WPC Product Bundles - Creates bundle relationships while maintaining inventory

# Support

For support, please contact MoizWordpress Development Team. We're happy to help with any questions or issues you may have.

# Changelog

- Version 1.0: Initial release

# Contributing

If you'd like to contribute to the development of this plugin, please fork the repository and submit a pull request. We appreciate your feedback and contributions!

# Email: 

[email protected]

# WhatsApp

+92-304-4887447




*/