Your extension works. Now charge for it.

What you do not have to write

Every one of these is a day you already know how to spend, and none of them is your product.

• A licence check that survives

  A network failure is recorded as "can't tell", never "invalid". Fourteen days of offline grace, and a downgrade only after the window expires and three checks in a row fail. Dropping a paying customer during a provider outage is the worst thing this code can do.

• MV3 that stays alive

  Service workers are torn down constantly, so re-checks run on alarms rather than timers, and every piece of state lives in storage. The popup, options page and content script all ask one place for the plan.

• Two payment providers

  Polar and Lemon Squeezy behind one interface. Switching is the string on line 9 of lib/config.ts. Adding a third means implementing three methods.

• A free tier that counts

  A daily quota with its own reset, spent with one await consume(1) before the work. Pro short-circuits it.

• The store paperwork

  Two permissions, not ten. Firefox's data collection declaration, required on new listings since November 2025. A sources zip for AMO. The manifest differences between the three stores live in one file, and today that file is 54 lines.

• The upkeep

  CI builds all three browsers and lints against AMO rules on every push. Dependency pull requests arrive on Mondays, already tested by the time you look at them.

The part that is harder than it looks

Deciding whether someone is on Pro sounds like a boolean. It is six cases, and the two in the middle are where the refund requests come from.

You would get here eventually. The question is whether you get here before or after the first customer emails you from a train.

What is in the box

Small enough to read, which is the point. No UI kit, no state library, no generator.

lib/
  config.ts        53 lines — the only file you edit
  license.ts       106 lines — the state machine above
  quota.ts         37 lines — free tier counting
  storage.ts       cache, on top of WXT storage
  types.ts         the LicenseProvider interface
  providers/
    polar.ts       Polar adapter
    lemonsqueezy.ts

entrypoints/
  background.ts    alarms, and the one place plans are answered
  content.ts       example, asks the background rather than deciding
  popup/           React — badge, quota meter, upgrade link
  options/         React — key entry, licence status

site/              this page and the documentation
wxt.config.ts      the only place the three browsers differ

Pricing

One payment. The version you receive is yours permanently, and updates keep arriving while your repository access does.

Payments run through Polar, which is the merchant of record and files VAT and sales tax. Full terms in the licence, which you can read before you buy.

Repository invitations are being sent by hand at the moment, while an issue with the automatic invitation is resolved with Polar. Expect access the same day rather than the same minute. If it has not arrived, open an issue at github.com/crxkit/support with your order ID and your GitHub username — that repository is public, so you can reach us before you have access to anything else.

What it is not

Licence validation in a browser extension runs on the client. Someone determined to bypass it will bypass it, and obfuscation only costs you debugging time. What protects revenue in practice is convenience and a steady update cadence, not a lock. If a capability genuinely must not be copied, it belongs on a server — and then the licence check is what authorises the request.

It is also not a UI kit, not a backend, and not your product. It is the paid-extension plumbing and the store paperwork, so that the part only you can write is the part you spend your week on.

Questions