We killed fifteen “proven” ideas, then killed most of the eight that were left. One survived — with a deadline attached. Here is the case, and the thing we built from it.
Brief No. 1 told the discouraging half of the story. A fifty-file corpus of easy-to-copy startup ideas, audited two ways: twelve of thirty-one load-bearing revenue claims failed verification, and zero of the fifteen top plays had a window still open. Then we ran full diligence on the eight conditional openings that survived. Six died on contact. One narrowed to a licensing play.
One survived intact, and it was the least glamorous item on the list: a flat-rate, monitor-only sales-tax product for small US online sellers. It survived because it is the only opening that cleared all five of our go/no-go gates and carries a dated event that opens the window on a schedule rather than on luck.
Since South Dakota v. Wayfair (2018), a state can compel an out-of-state seller to collect its sales tax once sales into that state cross an economic-nexus threshold — commonly $100,000, sometimes a transaction count. There is no physical-presence escape hatch anymore. A Shopify or Amazon seller can trip nexus in a dozen states in a good year and never notice, because nothing in their checkout tells them.
The consequence is not theoretical. In our Reddit demand-mining pass, the single highest-scored pain point across r/smallbusiness and r/ecommerce was a verbatim variant of “I forgot to collect sales tax for 18 months and got audited” — pain score 90 of 100. When a state discovers uncollected tax, the seller, not the buyer, owes the back tax, plus penalties and interest, out of margin already spent.
The affected population is large and structurally growing: Shopify and Amazon alone are roughly half of a $1.2T US e-commerce market, and the number of sellers tripping multistate nexus rises every year as marketplaces push them national by default. A 2023 Censuswide survey found more than 71% of businesses call the nexus rules “complex and confusing” and only 55% can fully explain their own obligations — the awareness gap is the demand. Market-size estimates for sales-tax software itself vary widely by source (roughly $1.6–6.0B), but all agree it is large and growing at double digits.
Sales-tax tools advertise a low base price, then meter the expensive part: they charge per return filed, per state, and per registration. For a seller in five states filing monthly — sixty filings a year — the honest all-in cost is not the headline number. It is several hundred dollars a month.
The seam is this: knowing where you owe and what to collect is the hard, high-value, error-prone part, and it is cheap to deliver as software. Filing is the low-value, occasional, mechanical part — and it is where the runaway fees live. Every incumbent bundles them. Split them, price the monitoring flat, and hand filing back to the seller or their bookkeeper, and you have a price no filing-inclusive competitor can match without cannibalizing their own filing revenue.
| Provider | Model | Files for you? | Est. all-in / yr |
|---|---|---|---|
| NexusRadar | Flat, monitor-only | No — by design | ~$348 |
| Quaderno | Calc-only subscription | No (filing = quote) | ~$1,188 |
| TaxCloud | Subscription + per-return | Yes | ~$2,100–2,600 |
| TaxJar (2026 pricing) | Sub + $50–55 / filing | Yes | ~$4,270 |
| Numeral | $75 / filing | Yes | ~$4,500 |
| Anrok | $100 / state / month | Yes | ~$6,000 |
| Avalara | Enterprise, custom | Yes | $7,400+ |
Representative all-in estimates from each vendor's published 2026 pricing for the stated profile (subscription + per-filing + registration where applicable); custom-quote vendors (Zamp) and usage-priced ones (Stripe Tax) are omitted as non-comparable to a flat scenario. Quaderno is the honest closest comparable — also calc-only, still ~3.4× NexusRadar. Figures vary by quote and volume; NexusRadar does not file returns.
The obvious objection: three products — TaxCloud, Kintsugi, Quaderno — already market themselves as the “cheap anti-Avalara.” True, and it is why the generic version of this play was rated closing, not open. The distinction that survives is scope: those products still bundle filing and still price it. The unoccupied configuration is explicitly monitor-only at a flat fee, sold as a feature (freedom from filing fees), not apologized for as a limitation.
The reason this is a build memo and not a someday-list entry is a date. TaxJar's Starter subscription is doubling from $19 to $39/mo, and its per-filing AutoFile fee is rising to $50–55 (from roughly $30–35) — confirmed on TaxJar's own pricing and support pages. Legacy month-to-month customers keep the old pricing only until 1 October 2026, after which they auto-convert; there is no permanent grandfathering.
That is a cohort of the most price-sensitive small sellers in the category, being repriced upward on a known day, with their switching intent peaking in the weeks around it. They are the exact buyer for a flat monitor-only tool. A migration moment with a calendar date is the rarest thing in the whole corpus: a window that rewards being early by construction, because the migration has not happened yet no matter how many people know the date is coming.
The fear with flat pricing is thin margin. It survives here because the deliberately-excluded piece — filing — is also the only piece with real variable cost and support load. Monitoring is near-zero marginal cost: threshold data is a compiled dataset that changes a few times a year, and rate lookups are a table read.
The diligence that killed six sibling openings applies here too. The real risks, in order:
The corpus's own lesson is that static analysis decays in weeks, so the honest response to a surviving opening is to build the cheapest real thing that tests it, not to write more about it. So we did.
NexusRadar is live. The free nexus checker is a genuine working tool: enter your sales by state and it evaluates each against that state's current economic-nexus threshold, flags where you've triggered or are approaching, and returns the rate to collect — entirely in the browser, no data transmitted. The flat-price positioning, the true-cost comparison, and the deliberately-honest “what it doesn't do” scope are all in place. It is a first-revenue MVP, not the finished product: live sales-channel sync (Shopify/Amazon/Stripe read-only), email threshold alerts, and per-state collected-tax summaries are the next build.
Pre-registered, so the decision isn't made in hindsight:
That last line is the through-thread from Brief No. 1. The durable asset was never the idea. It was the instrument — the gates, the calendar, the willingness to kill — and a product is just the instrument pointed at the one opening that survived it.