add a one-time setup fee to subscription?

Has anyone had experience adding a one-time setup fee to a subscription. We're subscription only. I was kinda thinking the setup fee would be a one time product auto-added to a subscription, but I'm a bit unclear how to make it one-time only. The Pay gem and Stripe docs refer to having about an hour between when a subscription is set up and when it's finalized but I'm not really understanding how it knows it's one-time only or how to do it automatically (the docs seem to indicate "oh it's easy to add it to a user's initial invoice!" in the sense of adding it by-hand)

I think I'm good on where to read up on it in Stripe, looking more for some guidance of how to handle such customization on the JSP side. Such as "in the subscriptions controller, create an after_action for only :new, which only gets fired once hen a subscription gets created, by which point your database will have been populated with a customer_id and a subscription_id which you can then modify as per the Stripe API. Here's where to write that json to send back to Stripe"