A self-serve copy example for this shopper question: Why did my discount not apply? Use it as drafting help for after the order summary and before support contact details without creating unsupported promises.
This is educational drafting material, not legal advice. Match the wording to the store's actual policy and operations.
Put the short version after the order summary and before support contact details. If the answer has exceptions, link to the full policy instead of trying to fit every rule into one sentence.
Good trust copy should reduce confusion without making the store sound bigger, faster, or more certain than it really is.