wc-subscribe-save.php

<?php

/**
 * Only copy opening php if needed
 * 
 * Adds a "subscribe and save" box after the add to cart button
 * Only added if `subscribe_save_id` field is set
 */
function sww_add_subscribe_save_to_product() {
	global $product; // get the current product to see if it has a 'subscribe and save'

	$subscription_id = (int) get_post_meta( $product->id, 'subscribe_save_id', true );

	// only proceed if we do have a 'subscribe and save'
	if ( $subscription_id ) {
	
		$subscription = get_post( $subscription_id );
		
		// output our subscription short description + the add to cart shortcode
		echo wp_kses_post( $subscription->post_excerpt );
		echo do_shortcode( '[add_to_cart id="' . $subscription_id . '"]' );
	}
}
add_action( 'woocommerce_single_product_summary', 'sww_add_subscribe_save_to_product', 31 );