There isn’t, but let me give some more information on charges that might help alleviate concerns. I’ll refer to our pricing page.
Firstly, you’re on the legacy Hobby plan, which means you have no monthly subscription fee - as opposed to the (non-legacy) Hobby plan which has a $5/month fee.
It sounds like you’re running app(s) within our free allowances? If that’s the case, as long as you’re not provisioning resources beyond the free allowance, you basically don’t need to worry at all about getting charged. For example, if you have one shared-cpu-1x with 256mb RAM running, there’s simply no way for that setup to exceed our free allowance.
One caveat to that is outbound bandwidth, which you don’t necessarily have full control over. If your app receives abnormally large amounts of traffic, we’ll email you. If you end up with unexpected outbound bandwidth charges because of abnormal traffic, email billing@fly.io. We have refunded many people in a similar situation; we’re not interested in making money from surprises like this.
The other caveat is billing changes, e.g. upcoming rootfs billing. We will absolutely email you to let you know about these and explain how you might be affected/how to avoid being charged.
Ultimately, that might still not be enough reassurance, and I totally understand that. The request for card details is a fraud prevention measure that we’ve had to crank the dial on a bit recently, to deal with various fraudulent usage of our free allowances. I will say that I’m surprised your account was flagged, and I’ve initiated an internal discussion regarding your specific situation.