You get billed monthly, so really, letting it go for a few months is on you.
> The one thing that really pissed me off was how easy it was to set up vs how hard it was to take it back down > You have to install the SDK and perform some (seemingly complex) commands.
Hu, no it's not. Really it's litterally one click to shutdown an RDS instance, and always has been.
> But they wanted this huge list of things from me in order to refund it, like a super in-depth explanation of how it happened.
I mean that makes sense to me. They did reserve and partially used these resources for you, so it's only fair that you have to go through the trouble of explaining why they would let it go. If there's no downside everyone would just reserve a bunch of resources all the time.
From your comment, it seems you didn't even bothered to answer their questions to get a refund, I would totally not hesitate to charge you if I was in AWS place.