Stripe "Subscribe the user" for existing customer


I am having a problem where I have two sets of payment systems, one using stripe’s addon function “Subscribe the user …” and one for one-off payment using Stripe.js 2’s plug-in.

The problem I have is if the user purchases a one-off item first, the workflow creates the user in stripe and assigns a customer-id to the user, which all works fine.

But if the user then goes and subscribes to a plan, there is no way within the action to identify the customer with the existing customer-id, instead, the subscription action creates a new customer. in stripe.

Is there any way other than building the subscription function from scratch to avoid this problem?