One code applies, a second is rejected, and a third combines with both. The rules governing which discounts stack are consistent once the funding behind each is understood.
Discounts have different funders
A manufacturer coupon is reimbursed by the brand, a store coupon is funded by the retailer, and a card-linked offer is funded by the issuer or the network.
Because the money comes from separate parties, combining a manufacturer coupon with a store coupon costs the retailer only its own portion, which is why that pairing is commonly allowed.
Combining two retailer-funded discounts doubles the retailer's cost on a single item, which is why store coupons almost never stack with each other.
The order of application changes the result
Percentage discounts applied before a fixed-value coupon produce a different total than the reverse, and systems apply them in a defined sequence.
Most platforms apply item-level discounts first, then order-level ones, then payment-linked rebates, which means a percentage code usually acts on the already-reduced price.
Tax and shipping calculations follow that sequence too, and in many states sales tax is calculated on the price after retailer discounts but before manufacturer reimbursements.
Exclusions protect thin margins
Codes routinely exclude clearance, specific brands, gift cards and certain departments, and those exclusions cluster where the margin cannot absorb a further cut.
Brands also impose minimum advertised price terms, which prevent a retailer from displaying a price below an agreed level and therefore restrict which discounts may apply.
The exclusion list is where most stacking attempts fail, and it is usually longer than the promotional message that carried the code.
Technical limits shape what is possible
Many e-commerce systems accept only one promotion code per order, which is a platform constraint rather than a deliberate policy.
Loyalty discounts, automatic cart promotions and payment-method offers often apply outside that single code field, which is why they combine when codes do not.
Retailers with older systems sometimes cannot allow stacking even where they would want to, and their promotions are designed around that limitation.
Published policies are the reliable source
Stacking behavior varies enough between retailers that experience with one is a poor guide to another, and it changes when platforms are replaced.
Most retailers publish coupon terms describing what may be combined, and grocery chains in particular maintain detailed policies because manufacturer coupons are central to the category.
Reading that policy resolves in a minute what testing codes at checkout resolves slowly and often incorrectly.