Discount stacking rules
The logic deciding which promos can combine on one order, in what sequence they apply, and when one blocks another.
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What it is
Stacking rules answer two questions: which promotions may combine on one order, and in what order they apply. Platforms commonly sort discounts into classes (product discount, order discount, shipping discount, automatic versus code), and a popular policy is to allow at most one from each class. That is a policy, not an industry rule: what actually combines varies a lot between platforms and between merchants on the same platform, and some setups happily take two order-level codes. Go read your own combination settings rather than assuming.
Order of operations is the part people miss. '20% off' then '$10 off' on a $100 cart leaves $70; '$10 off' then '20% off' leaves $72. Pick a sequence, write it down, and make sure the cart, the checkout, and the refund calculation all use the same one, or your returns will hand back money you never took.
Gotcha: the floor. Sale price plus a stacked percentage plus free shipping can quietly sell below landed cost, and the shoppers who find that combination are exactly the ones who share it. Enforce a minimum margin or a maximum total discount per order, and decide up front whether stacked discounts apply before or after tax and shipping.
Ask AI for it
Implement discount stacking rules for my cart. Classify every discount as product-level, order-level, or shipping-level, and as automatic or code-entered. Allow at most one order-level discount per order: when several qualify, pick the winner by a configured priority value, falling back to greatest customer benefit on ties, and record every rejected candidate with the reason so support can explain it later. Then apply in a fixed sequence: product discounts first (per line item), then the single chosen order-level discount, then shipping discounts. Add a per-discount 'combines with' allowlist and a hard cap so total discount never exceeds a configurable percentage of subtotal. Show the shopper each applied discount as its own line with its own amount, and when a code is rejected, say which existing discount it conflicts with.