The app tried to charge me for the usage from Oct 1-Nov 1

that is the charge for your usage during september

no the estimated bill is just how much it thinks your projects will cost you. it is paid at the end of the month

oh okay

and how do I check previous monthly usage?

I'm not sure. @Brody dyk if this is possible?

its not, the invoices for the previous months only contain what you where charged for like the amount of cpu and mem

it would be far too much for railway to keep

Update: You can now view usage of previous billing cycles, you'll see the option in your account usage page