Sining up for the free trial will give you 365 days of time and a free budget of 300$ to spend on computing time. After this your Card WONT be charged. You will get notified and if you do not do anything the machines will simply shut down. Therefore this is pretty save concerning the $$$ :).
I looked into using AWS briefly, but it didn't make sense numbers wise, plus it's really easy to forget about it, then you're credit card statement shows up
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Per above, "After this your Card WONT be charged," BUT the LTT post is from Oct 2019, so maybe there's an "accident" or policy change and they charge your card anyways.