InvoiceKit made every displayed total a testable product claim. Its invoice detail brings the record identity, customer context, line items, and completed total into the same review surface, so the calculation does not hide behind a dashboard number. That was an important delivery decision for a data-heavy app. When accuracy matters, place the inputs close enough to the output that a reviewer can reconcile them.

While building InvoiceKit, I stopped reviewing business features as a list of screens and used five questions: Who owns the record? What can change? Where is it summarized? Which state leaves the app? How is persistence verified? For an invoice, those questions connect owner-scoped Firebase access, revisions and statuses, dashboard and detail views, and PDF/CSV sharing. If any answer is vague, the estimate still contains hidden work.