Stripe can be configured to send emails to your customers when a failed payment goes through. On this email there is a link for the customers to update their payment information. This link can be set to the page where you have the Billflow Subscription Management Portal, and from there they will be able to update their payment details. Follow the steps below to enable that.
Go to https://dashboard.stripe.com/settings/billing/automatic & turn on Customer Emails for failed payments.
Click on the eye icon and paste in the URL where you have your Subscription Portal embedded.
Done! You're all set. Now when a customer has a failed payment, they'll receive an email with a link to update their payment information.