Case study 01A · Financial SaaS
Keeping financial state trustworthy across systems.
At TEO, I own the payments and accounts-receivable path from invoice creation through hosted checkout, asynchronous processing, and the product state users ultimately trust.
- TypeScript
- QuickBooks
- Payabli
- Webhooks
- Background workers
- Design tokens
- WCAG
- 01 · Role
- Lead Full-Stack Engineer
- 02 · Domain
- Property-management finance
- 03 · Focus
- Payments, AR, product systems
01 · Problem
The screen was not the hard part.
A financial product cannot treat each integration response as its own version of the truth. QuickBooks, Payabli, webhooks, and background workers all observe or change a transaction at different moments.
The engineering problem was to preserve a coherent account of the invoice while keeping the interface useful, the payment path conservative, and the product foundation enforceable across the team.
02 · System
One money path, three engineering layers.
The product separates external events, durable financial state, and interface behavior so that each layer has a clear responsibility.
- 01
Invoice and ledger state
QuickBooks invoicing feeds a product model where invoice status is derived from the ledger rather than maintained as an unrelated flag.
- 02
Payment event processing
Payabli hosted checkout, webhook handlers, and background workers carry payment activity through asynchronous boundaries without treating an early response as final state.
- 03
Enforced product foundations
A three-layer token architecture, ESLint enforcement, and WCAG remediation give the interface system rules that can be checked instead of remembered.
03 · What made it hard
The money path has to fail closed.
Correctness here is not only about a successful transaction. It is also about refusing unsafe transitions and making discrepancies visible before they become financial state.
Default-deny safeguards
Ambiguous or incomplete financial conditions do not earn a permissive fallback. The system protects the boundary before dispatching a money-moving action.
Adversarial review
A multi-model AI review gate challenges assumptions in the payment path and supplements conventional engineering review on high-risk changes.
Accessible by construction
WCAG remediation and enforceable design tokens keep accessibility tied to the product system, not reserved for a cleanup pass after release.
04 · Next