It is impossible to spend any real money (as opposed to their $300 credit) without switching to a paid account, which is a multi-step process that you can't perform accidentally.
After the 30 day trial expires your account is switched to the "always free" mode. It does not allow creating any paid resources which are not marked as "always free".
I was curios and read about it, and you are correct. But just like I said, there are many pitfalls. for example - I assumed you can change your paid plan freely but:
It's with HostUS. I got a coupon a few years ago (via LowEndBox), and it's been renewing at the same rate ever since. I have no complaints about them, but I'm not sure you can get the same pricing today.
At this price point you are barely paying for the IPv4 address. It will be an OpenVZ container.
I had a $15/yr VPS with BuyVM.net for many years and would absolutely recommend them at this price point, except that they have shut down this offering and switched to KVM (it's for the best). Ramnode.com are honest enough and still offer the "192MB SVZ" plan for $15/yr.
I would rather scrape by on the GCS/AWS/Heroku free tier, Netlify / GH pages, ...rather than going back to OpenVZ. Better to pay just a few dollars more for a proper KVM VPS.
https://www.oracle.com/cloud/free/#always-free
Or four ARM VMs. Nice. They didn't have those when I signed up.