Hosted sales-tax API built on open-source rate engine

What to build

A hosted, self-serve REST API (plus a Shopify/WooCommerce plugin) that wraps OpenTaxRate's open-source street-address-level US sales tax engine, so small e-commerce sellers and indie SaaS founders can get accurate tax rates via API key and usage-based billing instead of signing an Avalara or TaxJar enterprise contract.

OpenTaxRate already solved the hard data problem of resolving US sales tax to the street-address level; the opportunity is in the thin, trustworthy packaging layer — a cheap, self-serve API and e-commerce plugin that small sellers can adopt in minutes.

Demand

Small e-commerce sellers and indie SaaS builders who find Avalara and TaxJar too expensive, too enterprise-sales-heavy, or too complex for their low transaction volumes are the target buyer, and OpenTaxRate's Show HN launch signals a fresh, free data source they could build on right now.

• Hacker NewsShow hn

  Show HN: OpenTaxRate – open-source US sales tax rates by street address — 4 points, 0 comments on HN. Builder says they made it after trying many sale[s tax APIs].

• Hacker NewsCommunity signal

  Project explicitly framed as an open-source alternative in a market dominated by paid enterprise tax compliance vendors, implying direct positioning against Avalara/TaxJar-style pricing.

• Avalara / TaxJarAdjacent paid tool

  Existing paid sales tax API vendors (Avalara, TaxJar) sell primarily to mid-size and enterprise merchants with sales-led, quote-based pricing, leaving small sellers and indie SaaS companies underserved by a cheap self-serve option.

Stack

• OpenTaxRate (GitHub engine)
• FastAPI or Express wrapper API
• Postgres for jurisdiction/rate caching
• Stripe metered billing
• Shopify App API / WooCommerce REST API
• Redis for request caching and rate limiting

Solo + AI difficulty

Wrapping OpenTaxRate behind an authenticated API with API keys and Stripe usage billing is straightforward and AI-codeable in days; the harder, ongoing work is keeping tax jurisdiction data accurate over time and building enough trust messaging (sourcing, update cadence, disclaimers) that sellers feel safe using a rate-only tool. Realistic time-to-MVP is about 1-2 weeks for the API alone, 3-4 weeks if a Shopify app listing is included.

Entry threshold

Needs a Postgres plus PostGIS hosting setup and a geocoding pipeline, but the hard part (state-by-state rate adapters) is already open-sourced, so a solo builder can ship a billed API wrapper or storefront plugin in a few weeks; ongoing maintenance of changing state tax rules is the real recurring cost, not capital or licensing.

Window

ongoing, no acute deadline

Where to find first users

• Indie Hackers forum
• r/ecommerce and r/shopify
• Shopify App Store listing
• Follow-up Show HN or Show IH post once the wrapper is live

Competitors

• Avalara
• TaxJar
• Vertex
• TaxCloud

Counter-signals & risks

• US sales tax determination requires continuous maintenance of thousands of overlapping and frequently changing jurisdiction rates and rules across state, county, city, and special districts, which is costly to sustain reliably as a small or indie operation and is a core reason incumbents charge enterprise pricing.

• Avalara and TaxJar bundle filing, remittance, exemption certificate management, and audit liability/compliance guarantees beyond rate lookup, so a rate-only API may not be a sufficient substitute for sellers with real compliance exposure.

• An open-source project's data accuracy and update cadence are unproven at launch (Show HN stage, 4 points, 0 comments), and errors in tax calculation carry direct financial and legal liability for adopting merchants.