How discount codes actually behave at checkout
Most codes fail for one of four reasons, and all four are visible before you reach the payment step.
A code that does not apply is almost never a broken code. It is usually a basket that does not match the code’s conditions, and the store rarely tells you which condition failed.
The minimum basket is counted after instant discounts
Stores compute the minimum on the item subtotal after any automatic markdowns, and before tax and shipping. A basket that reads 160 at the top can be 140 by the rule the code applies.
Full-price exclusions are the most common trap
A large share of codes exclude items already reduced. If half your basket is on sale, the code may apply to the other half only, or refuse entirely.
Codes are case-sensitive, and whitespace counts
Pasting from a page often carries a trailing space. If a code looks correct and still fails, retype it by hand once before concluding it is dead.
One code per order, nearly always
Stacking is rare and usually deliberate on the merchant’s part. Where we know two codes stack, the offer says so explicitly.