Two assertions encode the reference merchant's currency and tax treatment

[BrocksiNet]

Follow-up to #101. Two assertions hold only for a merchant that trades in a particular currency and charges no tax.

1. The idempotency conflict payload is a no-op for EUR merchants

idempotency_test.py::test_idempotency_create sends a request, replays it, then sends a "conflicting" one under the same key expecting 409:

conflict_payload = create_payload.model_copy(deep=True)
conflict_payload.currency = "EUR"

The original payload's currency comes from conformance_input.json. For a merchant that declares EUR the conflict payload is byte-identical to the original, so a correct server replays it and answers 200/201 — exactly as the preceding assertion required. The test can only pass for a merchant that does not trade in EUR.

Patch derives a value that actually differs:
0006-idempotency-conflict-must-differ.patch

2. test_fulfillment_flow assumes the business charges no tax

expected_total = self.fixture_ctx.get_test_price() + option_cost  # base price + shipping

This asserts the grand total is item price plus shipping, which holds only where nothing else is added.

The suite models tax correctly everywhere else, and contradicts itself here:

• totals_test.py lists tax and fee in _ADDITIVE_TYPES as valid additive entries
• business_logic_test.py verifies the total by summing every non-total entry

So a merchant charging VAT — ours is a German business at 19% — passes the arithmetic checks in both of those and fails this one.

Patch sums the entries the way business_logic_test already does:
0007-sum-totals-instead-of-assuming.patch

Both verified: with these applied, idempotency_test and fulfillment_test go green against an independent merchant.