What worked for me was handing them a credit card and transitioning myself out of the free tier. (I'd use the free credits they offer prior to doing this - they give you something like $300 immediately on signup.)
The always-free infra remains free, you just have the chance of incurring a bill if you make selections that aren't free or exceed block storage/egress (200GB/10TB) limits of the always-free tier. Leaving the free/trial tier gives you access to a much larger pool of instances. I never successfully deployed an A1 instance prior to becoming a "paying" customer - now I've done it hundreds of times without ever having an issue.
I've been running a small k0s cluster and a standalone webserver for months while incurring about $2.50 - $3 in spending each month, primarily from being slow to remove instance snapshots sitting in block storage.
Even things that are oddly expensive on AWS - like NAT - are free on Oracle. There are zero gotchas.
I hit the same roadblock as the above user and it never occurred to me to just cross the barrier with cash and then scale back to free. Thanks for this.
