Automatically add a coupon / global discount to a WooCommerce cart.

woocommerce-global-discount.php

function global_discount(){
	global $woocommerce;
	
	$coupon = 'your-coupon-code-name';
	
	// We'll add the coupon code the first time.
	// This way users can remove the coupon code
	// when they have a bigger discount coupon.
	if( 0 == $woocommerce->cart->cart_contents_count ){
	// This coupon can be managed from the coupon dashboard
		if( !$woocommerce->cart->has_discount( $coupon ) ) {
			$woocommerce->cart->add_discount( $coupon );
		}
	}
}
// Whenever a users adds a product to the cart.
add_action( 'woocommerce_add_to_cart', 'global_discount' );