That's a good point. Most subscription service providers, like Stripe, Chargrbee, Braintree, etc, use a fairly conventional one-to-many data architecture for Customers and Subscriptions.

Just take care to use the subscription service provider data model how it is intended. It is possible to design your integration in a way that goes against the grain and end up with gaps in your data. For example, by re-using a single subscription instance per customer and changing it's properties when the customer down/upgrades rather than creating a new Subscription instance.