Modify WooCommerce Subscriptions Subscription Strings

functions.php

<?php
add_filter('woocommerce_subscriptions_product_price_string', 'modified_subscription_strings', 10, 3);

function modified_subscription_strings($subscription_string, $product, $include){
	switch($product->get_slug()){
		case 'green-magazine-subscription':
		case 'classic-farm-tractor-subscription':
		case 'little-green-magazine-subscription':
			return '$' . number_format($product->get_price(), 2, '.', '') . ' / year';
			break;
		default: 
			return $subscription_string;
	}
}