A collection of recipes to force add a single product to the WooCommerce cart and redirect from the shop, single product, and cart pages right to checkout.

my_single_product_auto_discount_woocommerce_shop.php

<?php
/**
 * Always add my single product by ID to the cart if it isn't already there.
 */
function my_woocommerce_force_add_product_to_cart() {
	$product_id = 325;
	$product_cart_id = WC()->cart->generate_cart_id( $product_id );
	if ( ! WC()->cart->find_product_in_cart( $product_cart_id ) ) {
		// Yep, the product with ID is NOT in the cart, let's add it then!
		WC()->cart->add_to_cart( $product_id );
	}
}
add_action( 'woocommerce_before_checkout_form', 'my_woocommerce_force_add_product_to_cart' );

/**
 * Redirect single product pages in WooCommerce to the cart page.
 */ 
function my_woocommerce_redirect_product_pages() {
	if ( is_admin() ) {
		return;
	} elseif (
		function_exists( 'is_product' ) && is_product() ||
		function_exists( 'is_shop' ) && is_shop()
	) {
		wp_safe_redirect( wc_get_checkout_url() );
		exit;
	} 
}
add_action( 'wp', 'my_woocommerce_redirect_product_pages', 99 );

/**
 * Skip the WooCommerce cart page and go right to checkout.
 */
function my_woocommerce_add_to_cart_redirect() {
	return wc_get_checkout_url();
}
add_filter('add_to_cart_redirect', 'my_woocommerce_add_to_cart_redirect');

/**
 * Remove the WooCommerce coupon code message on checkout page.
 */
function my_woocommerce_coupon_message( $msg, $msg_code, $instance ) {
	if ( is_checkout() ) {
		return '';
	}
	return $msg;
}
add_filter( 'woocommerce_coupon_message', 'my_woocommerce_coupon_message', 10, 3 );