Use Stripe's restricted API keys
Stripe has restricted API keys that allow Stripe's customers to give a subset of the permissions to a key that it can then share with a 3rd party service.
From Stripe's documentation https://stripe.com/docs/keys#limit-access
Limiting access with restricted API keys A restricted API key allows
only the minimum level of access that you specify. Restricted keys
cannot interact with many parts of Stripe’s API and are intended to
reduce risk when using or building microservices. They should not be
used as an alternative to your account’s API keys during development
of your Stripe integration.
Use restricted API keys if you’re working with microservices that
interact with the Stripe API on your behalf. You can create restricted
API keys that limit access to, and permissions to specific account
data. For example, you can create a restricted key that grants
read-only access to dispute data, then use it with a dispute
monitoring service.
STORE RESTRICTED API KEYS / ENCOURAGE KEY ROTATION
These restricted keys are much less of a security risk than a full API key which is akin to "root" access. Encrypted storage of these restricted keys could meet your security needs. Also educating your customers of best practices including API key rotation would help reduce and share risk with your customers.