Use the current selected base
One of each dated reference totals£66.99:£23 notebook,£19.99 boxed pen and£24 chocolate category quote. This is a comparison base across merchants,not a single accepted order. Update prices and whole offer quantities to match the actual selected purchase. Include the optional notebook box only when it is actually selected and quoted,not by quietly changing the base product’s contents.
Exact full credit is a valid boundary
A confirmed merchandise-only credit of£66.99 against that base leaves£0 merchandise payable. In a hypothetical test,shipping£4.99 and additional tax£1.25 remain payable as£6.24. Those test charges are not claimed seller fees. The calculation must not remove shipping just because the merchandise was fully credited or add VAT again when the ticket already includes it.
One penny over is unresolved
A£67 credit against£66.99 is rejected rather than producing a negative purchase total. Blank or negative prices,exponent notation,fractions of a penny and values beyond the supported range are also rejected. Counts must be whole nonnegative numbers. These are meaningful bounds for a purchase ledger,not cosmetic validation that accepts every typed value.
Check ordinary decimal cases
The tool converts supported amounts to integer pence before multiplying and subtracting. Tests include£47.99 ×2 =£95.98 and£0.10 ×3 =£0.30,with exact full credits leaving zero merchandise. It also tests maximum values without exceeding safe integer arithmetic. A per-offer display is separate from an inner-unit comparison;paper sides and chocolate pieces do not become complete gifts.
Leave missing charges blank
Unknown shipping,tax or other-charge fields prevent a final total. Enter zero only when confirmed. The tool runs locally and does not place an order,take payment,send a message or reserve goods. Actual merchant approval,invoice treatment and workplace purchase approval remain outside the calculator. Independent gifting,food and content review remains pending.
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 gifting/food/content/commerce review remain pending.
Papier Stellar Green hardback lined notebook41793 ↗
Ryman Parker Jotter gift-box pen0870220085 ↗
Hotel Chocolat Thank You Everything18-piece263518SL14 ↗
Hotel Chocolat current UK Thank You category quote ↗