I get you completely—it’s really annoying to have a debt on something that you planned not keep using, and especially if you were only trying out.
With regard to the beginners, shopify assumes user’s manual cancellation so this may not make sense in some cases.
Considering that you are unable to make the payment of $58 now, the best thing for you to do is plead with Shopify for mercy through its support channel.
Just tell the truth about what happened with you: you made trials, failed to comprehend how one can opt out of it safely then now money due because of lack of enough funds.