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 .
 
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.

So 11 days cost $99 out of that $300 starting credit?

I'm looking at his 1 year free trial that started 11 days prior to posting and showed 99 already spent.

Here is an example from the calculator.

 

The point was to give everybody a chance to boost their PPD by using the free $300 credit.  If you want to continue using the cloud, its up to you. Its another way to fold without investing into a bunch of hardware.  I'm not a number cruncher but maybe someone can do a cost analyst vs all the up keep at your home.  Examples: Hardware and Electricity cost.  Just some food for thought.