add-to-cart-one-product.php

<?php
/**
 * Redirect directly to checkout page only for ONE product
 * if cart contains $product_id then go directly to checkout.
 **/
add_filter ('add_to_cart_redirect', 'redirect_to_checkout');
function redirect_to_checkout() {
        global $woocommerce;

		//Get product ID
		$product_id = apply_filters( 'woocommerce_add_to_cart_product_id', absint( $_REQUEST['add-to-cart'] ) );
		//Check if product ID is in a certain taxonomy
		if( $product_id = 6319){
					//Get cart URL
		    		$checkout_url = get_permalink(get_option('woocommerce_checkout_page_id'));
		    		//Return the new URL
		    		return $checkout_url;
		 };
}