WooCommerce Drip - Handle subscription / adding custom fields when order set to completed. This does not take into account the user's subscribe selection during checkout, and actually should be used instead of the subscribe checkbox.

gistfile1.php

add_action( 'woocommerce_order_status_completed', 'drip_subscribe_user_to_campaign', 20, 1 );
add_action( 'woocommerce_order_status_completed', 'drip_add_user_fields', 30, 1 );
function drip_subscribe_user_to_campaign( $order_id ) {

	$WC_Drip_Settings = new WC_Drip_Settings();
	$wrapper = $WC_Drip_Settings->wrapper();

	$order = wc_get_order( $order_id );
	$email = $order->billing_email;

	$api_key = $wrapper['api_key'];
	$account_id = $wrapper['account'];
	$wcdrip_api = new Drip_API( $api_key );

	$params = apply_filters( 'wcdrip_checkout_subscribe_params', array(
		'account_id'	=> $account_id,
		'campaign_id'	=> $wrapper['subscribe_campaign'],
		'email'			=> $email,
	) );

	$wcdrip_api->subscribe_subscriber($params);

}

function drip_add_user_fields( $order_id ) {

	$events = new WC_Drip_Events();
	$events->new_order( $order_id );

}