Keep charging for express shipping on free-shipping orders

[sergical]

Free shipping over the threshold now covers standard shipping only, so express orders still pay for express. The threshold also moves into a named constant, and there are two tests for checkoutTotal.

Created with Claude Code

[github-actions]

Verdict: Approve with minor suggestions (no blocking issues).

Tests pass (18/18, 2 files). The actual diff is correct; it does not contain a duplicated shippingCharge line — that artifact only showed up in a note I added for one of the sub-reviewers transcribing the diff and does not exist in fixtures/latest.diff or the final file content, so disregard that part of the correctness pass.

Correctness

• The core logic change is intentional and correct: freeShipping = subtotal >= FREE_SHIPPING_THRESHOLD && !cart.express makes express orders always pay shipping, even above the $100 threshold. This is a genuine behavior change from the previous rule (any order ≥ $100 shipped free, express included). It's covered by the new tests and passes, but it is a customer-facing pricing change — worth a one-line comment or changelog note confirming this is the desired business rule, not an accidental regression.
• Test coverage gap: no test exists for an express order under the threshold (should still charge, same as before) or for the exact boundary (subtotal === 100). Adding these would lock in the intended behavior and guard the threshold comparison (>= vs >).
• Pre-existing, not introduced by this diff: no bounds-checking on taxRate or discount.percent, which could theoretically produce negative totals. Not blocking, just a note for future hardening.

Style

• freeShipping as a name now conflates "over threshold" with "not express." Consider renaming to qualifiesForFreeShipping or adding a short comment, since the express exception isn't obvious from the variable name alone.
• FREE_SHIPPING_THRESHOLD is declared between the Cart type and roundCents, away from its only use in checkoutTotal. Minor locality improvement: move it closer to where it's used, or group with other shipping-related constants if more appear later.
• Minor wording nit: the test title 'ships standard orders over the threshold for free' reads a little awkwardly — something like 'applies free shipping to standard orders above the threshold' would be clearer.

Overall this is a small, well-tested, low-risk change. The only thing worth confirming before merge is that excluding express orders from free shipping is the intended product decision.