Stacking has a reputation for being an advanced trick. In practice the rules are simple and consistent across most large retailers, and knowing them saves a lot of wasted checkout attempts.
What almost never stacks: two promotional codes
Nearly every retailer limits an order to one promotional code. The checkout usually replaces the first code with the second rather than telling you clearly, so check the order summary if you have tried more than one.
What usually does stack: a code plus a loyalty discount
Loyalty or membership pricing is applied to the item rather than the order, so a promotional code frequently applies on top. This is the most reliable stack available and it is why signing in before building a basket matters.
What usually does stack: a code plus store credit or a gift card
Gift cards, store credit and trade-in value are payment methods, not promotions. They are applied after the discount and reduce the amount charged to your card. Retailers rarely restrict this.
What usually does stack: a code plus a threshold reward
Free-shipping and free-gift thresholds are evaluated separately from percentage discounts. The catch is the order of operations: thresholds are usually assessed after the discount, so a large code can drop you below the qualifying subtotal. Apply the code, then check whether you still qualify.
What sometimes stacks: a code on already-reduced stock
A minority of retailers run explicit "extra x% off sale" promotions, and those genuinely do stack on marked-down prices. Outside those windows, sale stock is excluded from codes almost everywhere.
The order to do things in
Sign in first. Add eligible full-price items. Apply the single best code. Check the threshold rewards are still attached. Apply gift cards and credit last. Then read the final total before you pay, because that number is the only one that matters.