Senior Software Engineer - Money Movement

Simplex.; New York (NY); On-site; USD 220,000 - 300,000; Full time

Benefits offered by this job

Medical and dental benefits

401(k)

Catered lunches and dinners

Job summary

Simplex is hiring engineers to own the payroll and money-movement stack in a NY-based on-site role. You will design double-entry ledgers, idempotent payment execution, and reconciliations, while partnering with platform engineering and store owners who rely on accurate payroll data.

You will lead the financial correctness patterns and drive adoption across teams, including AI-guided coding and guardrails for money-touching code.

Qualifications

• 5+ years building production backend systems, preferably in TypeScript/Node.
• Fluency with transactional guarantees and idempotent processing.
• Deep PostgreSQL schema design and concurrent transactions.
• Experience with payroll, banking, or accounting integrations.
• Ability to design auditable, regulator-friendly ledgers and reconciliations.

Responsibilities

• Build our payroll product from the ground up with tax, withholdings, and filings integrations.
• Develop a bill-pay workflow for accounts payable within the platform.
• Embed bank accounts so clients access pygh revenues quickly.
• Own and harden QuickBooks integrations and month-end balance checks.
• Design ledgers for loyalty, gift cards, and deposits with correctness under concurrency.
• Define patterns for double-entry, reconciliation, and time-zone handling across teams.
• Scale AI-coding agents into the development workflow with safe guardrails.

Skills

TypeScript/Node

PostgreSQL

Financial systems

Idempotency

Ledger reconciliation

Payroll domain

Auditable ledgers

QuickBooks

AI tooling

Tools

Next.js

tRPC

Prisma

Vercel

ElastiCache

RDS

Job description

This role is on-site M-F in the Flatiron District, New York City, NY.

Our client is a Series A point-of-sale software company in NYC.

We move fast and ship daily. AI-assisted engineering isn't a talking point here; agents like Devin, Claude, and Codex are part of how the team works every day, and engineers who master that leverage ship multiples of what they could alone.

About the Team

Money Movement owns everything that touches money after the sale: Payroll, Banking, and Lending.

Today that means the accounting backbone our merchants reconcile their businesses on - QuickBooks sync for invoices, bills, COGS journal entries, and sales mappings - plus the ledgers behind loyalty, gift cards, and deposits, where being off by a cent is a bug. Next, it means payroll: our merchants run stores with hourly staff across multiple locations, and they want to pay their people from the same system that already knows every shift and every drawer count. This team is where we go from recording our merchants' money to moving it.

About the Role

You'll be one of the first engineers on our payroll build, and a technical owner of the financial systems where correctness is non-negotiable: ledgers, accounting sync, and the money movement rails underneath them. This means designing systems an auditor could read - double-entry ledgers, idempotent payment execution, reconciliation that proves everything ties out - and debugging from ambiguous production signals ("this merchant's books are off by $0.47 since the 9th") down to root cause. You'll partner closely with Platform Engineering, whose sale write path your ledgers consume, and directly with store owners whose payrolls you'll run. Beyond building, you'll establish the financial-correctness patterns the rest of engineering adopts.

In this role, you will

• Build our payroll product from the ground up - earnings computed from shift data the platform already has, tax withholding, payment execution, and filings integrations
• Develop our bill pay product that moves our customers from paying their suppliers by check to one-click handling accounts payable directly in our platform
• Embed bank accounts directly into our OS so that our customers have access to their card processing revenues in minutes
• Own and harden our accounting integrations (QuickBooks today), where a sync bug shows up as a merchant's books not balancing at month-end
• Design and maintain the ledgers behind loyalty, gift cards, and deposits - correct, idempotent, and auditable under concurrent writes
• Establish the financial-correctness patterns (double-entry ledgers, idempotency keys, reconciliation jobs, timezone cutoffs) that other teams adopt
• Multiply your output with AI agents: direct, review, and productionize agent-written code as a core part of your workflow - with the guardrails money-touching code demands

Your background looks something like this

• 5+ years building production backend systems, ideally in TypeScript/Node - though we care more that you write correct, reviewable code in any language
• Practical fluency with transactional guarantees: idempotency, atomicity, retry semantics, and what happens when a webhook arrives twice
• Real depth in PostgreSQL: schema design an auditor could follow, transactions under concurrency, and queries that stay fast as ledgers grow
• An instinct for the unglamorous parts of money - reconciliation, rounding, tax rates with three decimals - done exactly right
• A bias for root cause over patch, and a track record of turning one-off incidents into eliminated bug classes
• Comfort operating with high ownership in a small team where your decisions become the standard
• Experience with payroll, banking, ledger, or other financial systems where an auditor has actually read your data model
• Experience with accounting integrations (QuickBooks, NetSuite, Xero), payment processors, or ACH rails
• Familiarity with our stack: Next.js, tRPC, Prisma, Vercel, ElastiCache, RDS/RDS Proxy
• Experience directing AI coding agents (Devin, Claude Code, Cursor) and building the guardrails that make them safe at scale
• Retail or POS domain experience, especially multi-location operations or cash reconciliation
• History of establishing technical standards or architectural principles adopted across teams

Compensation

$220k - $300k + Meaningful equity

This range reflects the expected compensation for this role. Compensation within the range is determined based on experience, skills, and scope of responsibilities. In addition to base salary, we offer meaningful early-stage equity - this team's work is the foundation the company compounds on.

• Fully company-covered medical and dental benefits.
• 401(k)
• Catered Lunches and Dinners: We provide dinner each evening for employees who would like to eat and connect with the team. Lunch is provided on a weekly basis.