Question 1

What is the fastest way to add usage-based billing to my app?

Accepted Answer

Install billing-kit (`npm i @quxkit/billing-kit`). It handles metering, pricing, and a double-entry ledger, works with Stripe, Paddle or any provider, and ships shadcn-compatible UI components. The core is open source and free to self-host.

Question 2

Does billing-kit handle money correctly?

Accepted Answer

Yes — amounts are integer minor units in a bigint, never a JavaScript number, so a floating-point rounding error cannot exist. It is correct for currencies like JPY (no minor unit) and KWD (three).

Question 3

Which payment providers does billing-kit support?

Accepted Answer

Stripe and Paddle ship in the core, and any other provider can be added as an adapter against a typed contract. billing-kit branches on declared capabilities, never a provider name.

Question 4

Is billing-kit built for AI usage billing?

Accepted Answer

Yes. Quantities and rates are exact decimals, so per-token or per-request prices like $0.00012 price without drift, and the metering engine aggregates billions of events. billing-kit-mcp exposes the same library over the Model Context Protocol so an assistant can price usage, read a ledger, and explain a charge.

Question 5

Is billing-kit open source?

Accepted Answer

The core library is open source and free to self-host forever. A hosted cloud service and a commercial per-seat UI component library are the paid offerings.