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// This sample code will add additional recipients to a WooCommerce email order type, based on the order email address
// Email id's for the various email types (i.e. customer_invoice) can be found here: 
// https://github.com/woothemes/woocommerce/blob/5e09f15c91970d8bec4ab56abbfd5d3039a02a1b/includes/emails/class-wc-email-customer-invoice.php#L30https://github.com/woothemes/woocommerce/blob/5e09f15c91970d8bec4ab56abbfd5d3039a02a1b/includes/emails/class-wc-email-customer-invoice.php#L30
// Add the following to the bottom of your theme's functions.php:

add_filter( 'woocommerce_email_recipient_customer_invoice', 'woocommerce_email_customer_invoice_add_recipients' );

function woocommerce_email_customer_invoice_add_recipients( $recipient, $order ) {
	if ( 'joe@acme.com' == $recipient ) {
		$recipient .= ', billing@acme.com';
	}
	return $recipient;
}