I would like to get the product id for one-time payments, but I am unable find ANY webhook event that would give me the id of the product purchased.

I know that for subscriptions, invoice.paid event provides the product id, but one-time payments do not produce invoice.

I have read some suggestions such as appending product id to metadata of checkout session and listening for checkout.session.completed, but I am not sure whether that is a reliable source for the confirmation of the payment.

>Solution :

If you use Checkout for one-time payment, then yes checkout.session.completed is the event you want. However you will need to Retrieve and expand the Checkout Session using its Id. You can expand its line_items.data.price.product.