Putting this kind of type-based 'magic' in the code is a bad decision that will bite you very soon. It optimizes for being 'cute' rather than being clear and maintainable, and that's a trade-off that almost never pays off.
Here's the example that runs in hundreds of integration tests:
expect(billing_pricing_plans).to eq billing_plans(
1.month => [:free, :premium, :pro, :enterprise],
1.year => [:free, :premium, :pro, :enterprise]
)
bill = FactoryBot.create(:bill, products: [])
expect(bill.currency).to eq("USD")
But, in principle I agree billing, even the simple SaaS stuff, is much harder than most people expect it to be in 2025. My product (linked in the original article) is based completely on Stripe Billing - and it is still very hard to avoid all the footguns.
For people wondering, I even have an example how wrong it can go: I "audited" a successful SaaS I know uses custom Stripe billing. I paid $30 for a starter plan, but was able to "upgrade" to $2k plain for free. Here's the full video: https://www.youtube.com/watch?v=YuXp7V4nanU