Understand what the checkout is replacing

Apply the first offer and note the total before trying the second. Watch for a message saying the earlier code was removed. Some sites allow one promotional code but still apply a separate automatic reduction; others prevent the automatic offer from combining with any manual code. The terms need to confirm the particular arrangement.

A discount can also affect qualification for another benefit. If free delivery depends on the discounted goods total, a larger price reduction may trigger a delivery charge. If the threshold is measured before discounts instead, it may remain intact. Do not assume the order of the boxes on screen reveals the calculation.

Percentage reductions do not simply add together

Where two percentage discounts genuinely apply one after the other, the second usually acts on the already reduced amount. For a hypothetical £100 item, 20% off gives £80; a further 10% off that £80 gives £72. The combined reduction is £28, or 28%, not 30%.

That calculation only describes the stated example. An offer could instead apply to the original price, one selected item or a capped eligible amount. Read how the retailer defines it. If the wording is ambiguous, use the confirmed checkout total and ask for an explanation rather than treating a familiar calculation as a universal rule.

Include benefits outside the basket

Cashback, referral credit and loyalty rewards can have their own restrictions on voucher use. A code accepted by the shop might still make a cashback transaction ineligible. Check the separate reward provider's retailer-specific terms before counting both benefits.

Quidco's help pages, for example, identify unauthorised voucher use and retailer exclusions as possible reasons for a cashback claim being declined. That is a reminder to verify the actual combination, not a promise that any listed code guarantees payment.

Keep a short comparison of the choices: goods total, postage, immediate reduction and any conditional future reward. Prefer the option that works for your budget without depending on an uncertain payment later. Repeatedly adding codes without noting what changed makes it easy to finish with a higher total than the one you had initially.