OP here: Good question. We sit one layer above Stripe in the payments stack - we are payment gateway agnostics. When you need to manage many different pricing plans/upgrades our goal is to help you scale your subscriptions.

For example if you have a Stripe plan where you charge $5 a seat, but want to reduce the cost as the volume increases or you want to diversify the way you sell, maybe the subscriptions includes a usage charge billed monthly.