Numa — POS with CFDI invoicing built in

Stack

Impact

• Narrowed the product to a single market, which retired the multi-currency and daily exchange-rate machinery and moved CFDI 4.0 invoicing from a post-MVP phase to a pre-launch one.
• Catalog, inventory, POS, CFDI 4.0 invoicing and cash-and-finance all closed and tested; what remains before launch is SaaS billing, not the product.
• Invoicing ships inside the product — in Mexico a POS that cannot stamp CFDI does not compete with the established accounting suites.
• Self-serve billing on Stripe, against incumbents that still onboard and collect manually.
• Built entirely against the Firebase Emulator Suite, so the product reached a tested multi-tenant core at zero cloud spend before launch.

What I did

• Retired Venezuela support in a single refactor: the daily BCV exchange-rate job, the IGTF tax, USD/VES multi-currency, the onboarding country selector and the 'VE' branch all came out, and createTenant now seeds MXN and 16% IVA directly.
• Business dates resolve in the merchant's own timezone, configurable per tenant and defaulting to America/Mexico_City — Mexico spans four zones and a cash-out closed in the wrong one splits a day's takings across two.
• Multi-tenant isolation is enforced by Firestore rules with a dedicated test suite (24/24 passing), after an early review found a role-escalation hole where overlapping rule matches combine with OR.
• Backend-owned fields such as subscription state and tenant configuration are immutable from the client.
• Invitation-based onboarding with granular per-member permissions: acceptInvite requires a verified email, and any user-controlled text is escaped before it reaches email HTML.
• Session state never leaks between accounts — the tenant context is dropped when the uid changes and cleared on every logout.
• Firebase SDKs load through lazy getters rather than eager imports; the change cut the initial bundle from 654 KB to 225 KB and keeps it near 290 KB as features land.
• Every phase closes with a multi-agent code review before moving on: 16 findings fixed in Phase 0, 13 in Phase 1.
• Combos price and discount as a unit but consume as a recipe: selling one combo discounts every component from stock, so the shelf never lies because of a bundle.
• Subscription state is derived from dates, never from a stored field — active, grace and suspended are computed, so a failed write cannot leave a cancelled account looking paid.
• Global invoice for same-day sales without RFC, which is the case that actually breaks a Mexican POS at closing time.

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