Allow the exact merchandise zero
For one of each selected UK two-pack, the merchandise arithmetic is 5692 pence. A confirmed merchandise-only credit of£56.92 leaves exactly£0.00 merchandise. A£56.93 credit is above that subtotal and is refused. The tool does not generate a negative price or infer cash back from an excessive voucher.
Reject one penny above the subtotal
GBP inputs are parsed into integer pence with at most two decimal places, and package quantities must be whole numbers within the stated bounds. Two user-entered£47.99 packs total£95.98 exactly;three£0.10 packs total£0.30. These are arithmetic tests, not additional observed retail offers.
Keep shipping after a merchandise credit
A merchandise credit does not automatically cover shipping, tax or other actual charges. With test entries of£4.99 shipping and£1.25 other charge, the remaining total is£6.24 when all other charges are explicitly zero. These test values are not ESR shipping quotes or evidence of voucher eligibility. Already-included tax is not added again.
Refuse missing or invalid values
Blank charges keep the full total unresolved. Negative values, exponent notation, excess precision and above-limit amounts are rejected. No loyalty login, stock reservation, device approval or checkout submission occurs. Use the actual confirmed payable quote and fees rather than treating a public price observation as a final invoice.
A purchase case to keep explicit
If a credit note applies only to one selected product, the buyer must enter the actual eligible amount. The ledger cannot validate its scope. An excessive credit is refused rather than treated as cash back, and separate charges remain payable unless explicitly covered.
Sources & scope
Checked November 5, 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 retailer instructions. Current received labels and independent device/material/content/commerce review remain pending.
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