Online e-Gift Cards are valid for ONLINE use only, not valid in-store purchases.
After completing the purchase you receive two emails:
- A purchase receipt.
- Email with a link to claim and/or store e-Gift Card.
You need to forward the second email to the person you are purchasing it for; they will need to store the email for safe keeping, click on the link to print a copy of the e-card, or they can add the e-card to their Apple Wallet. Unfortunately this function is currently not available for Android devices.