For Robin

straight-to-checkout.php

/** Robin, you can add these lines to your child theme's functions.php file
 * If you're NOT using a child theme, please copy / paste this link 
 * https://github.com/woothemes/theme-customisations/archive/master.zip
 * and use that plugin to make custimizations like this one to your theme
 * Best wishes Robin
**/

add_filter ('woocommerce_add_to_cart_redirect', 'woo_redirect_to_checkout');
function woo_redirect_to_checkout() {
	$checkout_url = WC()->cart->get_checkout_url();
	return $checkout_url;
}