Flow Mobile uses Stripe to manage payments and subscriptions.
Stripe, Inc. is an Irish-American financial services and software as a service (SaaS) company which primarily offers payment processing software for e-commerce websites and mobile applications.
Millions of businesses of all sizes use Stripe's software to accept payments, send payouts, and manage their businesses online.
Some of Stripe's larger and more well-known partners are:
Stripe has been audited by a PCI-certified auditor and is certified to PCI Service Provider Level 1. This is the most stringent level of certification available in the payments industry. You can read more about Stripe's security on their information page - https://stripe.com/docs/security/stripe