It may be in the wrong box at checkout, mistyped, already spent, expired or disabled. Copying the code from the email, rather than typing it, rules out typos.
Many stores show it once the code is entered at checkout, before payment. Otherwise the store's team can look it up from the last few characters.
Usually, if the store can confirm who bought it. The team disables the old code and issues a new one with the remaining balance.
It depends on the store and the law where you sell. Many places limit or ban expiry dates on gift cards, so check before writing one into your policy.
Aide fills in the Shopify order and tracking, then drafts or sends the reply, as you choose.
