This one for completed initial subscription payments and subscription renewals.
function payment_made($subscription){
// How do I get the order details?
}
add
Some details can be obtained directly from the WC_ subscription object using the following methods
$subscription->get_id() //subscription id
$subscription->get_parent_id() //order id
$subscription->get_currency()
$subscription->get_customer_note()
$subscription->get_prices_include_tax()
$subscription->get_payment_method()
$subscription->get_payment_method_title()
$subscription->get_billing_first_name()
$subscription->get_billing_last_name()
$subscription->get_billing_company()
$subscription->get_billing_address_1()
$subscription->get_billing_address_2()
$subscription->get_billing_city()
$subscription->get_billing_state()
$subscription->get_billing_postcode()
$subscription->get_billing_country()
$subscription->get_billing_email()
$subscription->get_billing_phone()
$subscription->get_shipping_first_name()
$subscription->get_shipping_last_name()
$subscription->get_shipping_company()
$subscription->get_shipping_address_1()
$subscription->get_shipping_address_2()
$subscription->get_shipping_city()
$subscription->get_shipping_state()
$subscription->get_shipping_postcode()
$subscription->get_shipping_country()
$subscription->get_order_key()
$subscription->get_date_created()
$subscription->get_date_modified()