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Everything below is an invented example written to demonstrate arithmetic. No figure here reflects a real price, a real promotion or a real delivery charge, and nothing in this guide publishes a coupon code.
Start from a baseline
Take a short-trip list of three items and one delivery estimate:
- Jumper — £48 (example)
- Trousers — £54 (example)
- Walking shoes — £82 (example)
- Delivery estimate — £12 (example, unverified quote)
Basket before discount: £184. With delivery: £196. That is the number every offer must beat, and it is the number people forget once a discount is in front of them.
A fixed-amount discount
Suppose an offer gives £15 off orders over £150. The basket is already above the threshold, so no extra spending is needed, and the result is straightforward: £184 minus £15 is £169, plus £12 delivery, which is £181. The saving against the baseline is £15, or about 7.7% of the basket.
Fixed discounts are easy to compare because the saving does not change with the basket size. That also means their relative value falls as you add more: £15 off £300 is half the benefit of £15 off £150.
A percentage discount
Now suppose a second offer gives 15% off, capped at £20. Fifteen per cent of £184 is £27.60, but the cap applies, so the discount is £20. The basket becomes £164, plus £12 delivery, which is £176.
If the cap were £30 instead, the discount would be the full £27.60 and the total would be £168.40. A cap is the detail that decides whether a percentage offer beats a fixed one, and it is almost always stated in small print.
| Scenario | Basket | Discount applied | Delivery | Total paid |
|---|---|---|---|---|
| No offer (baseline) | £184 | £0 | £12 | £196 |
| £15 off over £150 | £184 | £15 | £12 | £181 |
| 15% off, capped at £20 | £184 | £20 | £12 | £176 |
| 15% off, capped at £30 | £184 | £27.60 | £12 | £168.40 |
All figures are illustrative. The point is the ordering of the columns: discount before delivery, total last.
The threshold trap
Now change the offer to £20 off orders over £220. The basket is £184, so you would need £36 more of qualifying items to use it. Spend £220, take off £20, add £12 delivery, and you pay £212 — for a basket you did not originally want. Compared with the £196 baseline, the offer has cost you £16 and added an item that was not on the list.
Product discount versus delivery discount
A delivery discount and a product discount are not interchangeable. Free delivery saves exactly the delivery charge, so against a £12 estimate it is worth £12. A 10% product discount on a £184 basket is £18.40, which is more — but only if the basket stays the same size. Two offers can look similar on a banner and behave very differently once the basket changes.
Record the two separately in your budget, then add them. Folding a delivery saving into an item price makes the item price wrong, and you lose the ability to compare the same item from two sellers.
Stacking is a terms question
Whether a product discount can be combined with a delivery offer depends on the terms of both, and it is not something you can infer from a checkout that happens to accept both. If the terms are silent, assume they cannot be combined and record the single best offer. If you plan around a combination that is later rejected, the budget was wrong for the entire planning period.
Keep the offer arithmetic next to the conditions in recording coupon conditions, and put the final figure into the "unconfirmed" column described in separating the parts of your budget. When the list is settled, the guide on reviewing and updating your spreadsheet shows how to clear out expired offers without losing the history.