Ask HN: Which tools do you use for SaaS customer management?

I trying to spin up a SaaS, so I need to track customer registrations, which plans they have subscribed to and what addons they use.

What is your approach to this?

My first idea was to build something on my own, but this is a problem every SaaS provider has, so there should be something pre-made out there. Like launchrock for gathering email addresses.