Most checkouts accept only one discount code, and many restrict which offers can be combined with it. Those limits are written into the promotions system for reasons that are largely arithmetic.
Discounts compound faster than they appear to
Two reductions applied in sequence do not add. Each is calculated on the price the previous one produced, and the combined effect is a larger reduction than the two figures suggest at a glance.
Add a member rate, a bulk offer and free delivery, and an order can fall below what the goods cost the retailer.
A single-code rule prevents that outcome without requiring anyone to check each combination individually.
The alternative would be modelling every possible pairing of live offers, which for a retailer running hundreds of promotions at once is not a realistic task.
Offers are funded from different budgets
A manufacturer-funded promotion, a store-wide sale and a loyalty reward are paid for by different parties with different agreements.
Combining them can mean one party unknowingly funding another's discount, which the underlying contracts usually prohibit.
The exclusion rules in a checkout often reflect those agreements rather than any judgement about how much the shopper should save.
The order of application changes the total
Whether a percentage discount is applied before or after a fixed-value voucher changes the final amount, sometimes noticeably.
Systems apply a fixed sequence, commonly item-level reductions first, then order-level codes, then delivery adjustments.
Since the sequence is fixed and not disclosed, the same two offers can produce different totals at different retailers.
It also determines whether a minimum spend threshold is assessed before or after the reduction, which decides whether an order qualifies for free delivery at all.
Returns complicate the arithmetic further
When part of a discounted order is returned, the refund must reflect the discount actually applied to that item rather than its list price.
Multi-offer orders make this calculation difficult, and it is a common source of refund disputes.
Limiting stacking keeps the reversal simple, which is a practical reason for restrictions that has nothing to do with margin.
Some combinations remain permitted
Cashback, payment card rewards and points typically sit outside the checkout entirely, so they combine with a code by default.
Delivery offers and gift-with-purchase promotions are often allowed alongside a discount, since neither reduces the item price directly.
Testing the permitted combinations before the basket is full avoids rebuilding an order around a saving that the system was never going to allow.