This snippet adds a subscription toggle to products with a linked subscription.

subscription-toggle.php

<?php
/**
 * Read the full post:
 * https://wptheming.com/2017/08/subscription-toggle-in-woocommerce/
 *
 * This snippet adds a subscription toggle to products with a linked subscription.
 */

/**
 * Adds an option under "Linked Products" on the product edit page.
 */
function example_product_options_related() {
    global $post;
    $meta_id = 'linked_subscription_id';
    ?>
    <div class="options_group">
        <p class="form-field">
            <label for="<?php echo $meta_id; ?>"><?php _e( 'Linked Subscription', 'example' ); ?></label>
            <select class="wc-product-search" style="width: 50%;" id="<?php echo $meta_id; ?>" name="<?php echo $meta_id; ?>[]" data-placeholder="<?php esc_attr_e( 'Search for a product&hellip;', 'example' ); ?>" data-action="woocommerce_json_search_products_and_variations" data-exclude="<?php echo intval( $post->ID ); ?>">
            <?php
            $product_ids = get_post_meta( $post->ID, $meta_id, true );
            foreach ( $product_ids as $product_id ) {
                $product = wc_get_product( $product_id );
                if ( is_object( $product ) ) {
                    echo '<option value="' . esc_attr( $product_id ) . '"' . selected( true, true, false ) . '>' . wp_kses_post( $product->get_formatted_name() ) . '</option>';
                }
            }
            ?>
            </select>
            <?php echo wc_help_tip( __( 'Enables customer to choose subscription option from product page.', 'example' ) ); ?>
        </p>
    </div>
<?php }
add_action( 'woocommerce_product_options_related', 'example_product_options_related' );

/**
 * Sanitizes and save the linked subscription product id
 */
function example_process_linked_subscription_meta( $post_id ) {
    if ( ! ( isset( $_POST['woocommerce_meta_nonce'], $_POST[$meta_id] ) || wp_verify_nonce( sanitize_key( $_POST['woocommerce_meta_nonce'] ), 'woocommerce_save_data' ) ) ) {
        return false;
    }
    $meta_id = 'linked_subscription_id';
    $linked_subscription_id = isset( $_POST[$meta_id] ) ? array_map( 'intval', (array) $_POST[$meta_id] ) : array();
    update_post_meta( $post_id, $meta_id, $linked_subscription_id );
}
add_action( 'woocommerce_process_product_meta', 'example_process_linked_subscription_meta' );

/**
 * Adds a subscription toggle option for products that have a linked subscription.
 * Including javascript inline for clarity.
 */
function example_add_subscription_option() {
    global $product;
    $display = false;
    $linked_id = $product->get_meta( 'linked_subscription_id' );
    if ( isset( $linked_id ) &&  $linked_id[0] ) {
        $linked_product = wc_get_product( $linked_id[0] );
        // If linked object is an actual product, set $display to true
        if ( is_object( $linked_product ) ) {
            $display = true;
        }
    }
    ?>

    <?php
    // Display the subscription toggle markup
    if ( $display ) :
        $onetime = array(
            'id' => $product->get_id(),
            'label' => 'One Time Order',
            'button' => 'Add to Cart',
            'price_html' => $product->get_price_html()
        );
        $subscription = array(
            'id' => $linked_product->get_id(),
            'label' => 'Subscribe and Save 15% (cancel anytime)',
            'button' => 'Subscribe',
            'price_html' => $linked_product->get_price_html()
        );
      ?>
    	<div class="subscription-toggle">
    		<div class="radio-select">
    			<fieldset>
    				<div class="option option-onetime-order" data-product="<?php echo $onetime['id']; ?>" data-button="<?php echo $onetime['button']; ?>" data-price="<?php echo esc_html( $onetime['price_html'] ); ?>">
    					<input class="sub-switch" checked="checked" name="onetime-order" type="radio" />
    					<label for="onetime-order"><?php echo $onetime['label']; ?></label>
    				</div>
    				<div class="option option-subscription" data-product="<?php echo $subscription['id']; ?>" data-button="<?php echo $subscription['button']; ?>" data-price="<?php echo esc_html( $subscription['price_html'] ); ?>">
                        <input class="sub-switch" name="subscription-order" type="radio" />
    					<label name="subscription-order"><?php echo $subscription['label']; ?></label>
    				</div>
    			</fieldset>
    		</div>
    	</div>

    	<script>
    	jQuery(document).ready(function($) {
    		$('.subscription-toggle .option').on( 'click', function() {
    			$('.subscription-toggle input').prop('checked', false);
    			var $option = $(this).find('input');
    			var product = $(this).data('product');
    			var button = $(this).data('button');
    			var price = $(this).data('price');
    			$option.prop('checked', true);
    			$('.single_add_to_cart_button').text(button);
    			$('.price').html(price);
    			$('input[name=add-to-cart]').val(product);
    		});
    	});
    	</script>
    <?php endif; ?>

<?php }
add_action( 'woocommerce_single_product_summary', 'example_add_subscription_option', 25 );

Thanks for the cool article.
FYI: $meta_id is not defined before this check

( ! ( isset( $_POST['woocommerce_meta_nonce'], $_POST[$meta_id] )

Hi,

Thanks for this great solution. Do you have something similar to work with Variable product?

Thank you.