carterapp — Personal Finance PWA

Problem

Most people stop tracking their spending because logging an expense takes too long. In Mexico there’s a second problem: money lives in pesos and dollars, and card cut-off and payment dates pile up around paydays.

Approach

Every decision was judged against one question: does this add a tap?

• One-screen entry — amount, category, and account on a single screen, with a keypad built for thumbs. Users can reorder the fields.
• Offline-first — every record is written to the device first (IndexedDB) and synced to Postgres later. IDs are minted on the device, so syncing twice never duplicates anything.
• Integrity in the database — composite foreign keys make it impossible for a transaction to disagree with its account’s currency or its category’s type. MXN and USD are never converted or summed together.
• Planning views — monthly budgets per category, every card’s cut-off and payment date on one calendar next to bills and paychecks, and a month-end balance projection that warns before the balance dips below zero.
• Platform — an installable PWA in Spanish and English, with a backend for subscriptions (Stripe) and push notifications.

Outcome

carterapp is live at carterapp.com.mx: a first expense is logged in under two minutes from landing on the site, and it keeps working on the metro or on a plane.