This is part of my PayPal smart subscription button script from sandbox environment. I am using the "custom_id" parameter for Identification, but not receiving in return in the IPN listener.
createSubscription: function(data, actions) {
return actions.subscription.create({
/* Creates the subscription */
plan_id: 'P-PlanId',
quantity: 5,
custom_id:101
});
Why are you using IPN? That is a very old and clunky service that predates the existence of the current subscriptions API by ~20 years.
For current Subscription integrations, use Webhooks. The PAYMENT.SALE.COMPLETED event is of particular interest.