Hizo Global Virtual Card

The problem

Hizo needed to let users across multiple African markets spend in USD online — subscriptions, SaaS, international merchants — without a traditional bank card. That meant issuing and operating virtual USD cards end to end: provisioning, funding, spend authorization, lifecycle (freeze/terminate), and secure card-detail reveal, all under fintech-grade security and compliance.

Constraints

• Third-party card issuer (Interlace) with its own SDK, webhooks, and failure modes — our system had to stay correct even when theirs was slow or inconsistent.
• Money movement — every state transition had to be auditable and idempotent; no double-issues, no lost funding events.
• Multi-market rollout with KYC gating before a user could ever hold a card.
• Mobile-first — the client was Flutter on iOS and Android, so the API had to be clean and the sensitive flows (card reveal) had to be secure on a device we don’t fully control.

What I built

I owned the system end to end — schema, REST API, security model, and the Interlace integration.

Flutter app ──HTTPS──▶ Hizo API (Laravel)
                          │
        ┌─────────────────┼───────────────────────┐
        ▼                 ▼                         ▼
   Card service      Funding ledger          Interlace SDK
   (issue/freeze/    (idempotent, audited)   (issuer webhooks)
    terminate)             │                         │
        └──────────▶ PostgreSQL ◀────── reconciliation ┘
                          ▲
                     Redis (rate limits, queues)

• A card lifecycle service modelling issue → active → frozen → terminated as explicit, audited state transitions.
• An idempotent funding ledger so retried requests and duplicate webhooks never move money twice.
• Webhook reconciliation against Interlace as the source of truth, so transient failures self-heal instead of drifting.
• A secure card-detail reveal flow (see the [end-to-end encryption case study] for the crypto envelope).

The decision that mattered

Treat our ledger, not the issuer, as the system of record for intent, and reconcile against the issuer asynchronously. This decoupled the user-facing flow from Interlace’s latency and let us prove correctness from our own audit trail — essential when the question is “where did this money go?”

Impact

• USD card issuance live across multiple African markets, on the iOS App Store and Google Play.
• A reusable issuance + ledger foundation other Hizo products (remittance, crypto) built on top of.

Source code is private (production fintech). Architecture and decisions are summarized here; happy to walk through detail in an interview.