Keep the observed subtotal exact
One selected$25.00tube,$17.99bag and$24.99bottle total$67.98in merchandise. The local basket parses decimal money into integer cents. It does not claim these items share a checkout, stock reservation or eligible promotion. Editable amounts are the user’s quotes for complete selected offers.
Allow exact full credit and reject excess
A$67.98merchandise-only credit produces zero net merchandise. A$67.99credit is rejected as one cent too large rather than silently capped. The example is an arithmetic test, not a coupon offered by any merchant. Actual promotion eligibility and exclusions need their current order terms.
Preserve charges that the credit does not cover
If shipping is$4.99and tax is$1.25with other charges explicitly zero, full merchandise credit leaves$6.24in this limited estimate. Blank or invalid charges keep the full total unresolved. The tool does not turn unknown shipping into free delivery or infer tax treatment from a product category.
Check meaningful small decimal cases
A user-entered$0.10price with quantity three gives$0.30. That exact credit is valid; $0.31is not. A user price of$47.99times two gives$95.98, so full$95.98is valid and$95.99is not. These are boundary test values, not observed dental-product prices. Formatting occurs after cent arithmetic.
Reject ambiguous input forms
Prices with more than two decimal places, negatives, exponent notation and values above the stated maximum are invalid. Package quantities are whole and bounded. Even a mathematically valid total needs an actual selected cart and current merchant terms before it becomes a real payable amount. The site has no order or payment form.
A credit must belong to the order being modeled
If a merchant issues a credit only for eligible items in its own store, it cannot automatically reduce the combined three-source ledger. Model that actual eligible order separately and retain the terms. The full$67.98example is an equality test, not an advertised promotion. Shipping and tax remain explicit charge fields, and unknown values do not become zero. This prevents a technically correct sum from being presented as an available deal whose eligibility and final order conditions were never established.
Sources & scope
Checked November 4, 2026. Manufacturer and retailer references support the specific statements. Dated displayed prices are not final checkout quotes, performance rankings or performance testing. This shopping reference does not replace the exact product label or qualified individual advice. Current received labels and independent veterinary-nutrition/dental/content/commerce review remain pending.
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