Available space is less and varies due to many factors. A standard configuration uses approximately 8GB to 11GB of space (including iOS and preinstalled apps) depending on the model and settings. Preinstalled apps use about 4GB, and you can delete these apps and restore them.
Not all devices are eligible for an Apple Store Gift Card. Payments are based on the received device matching the description you provided when your estimate was made. Apple reserves the right to refuse or limit the quantity of any device for any reason. Other restrictions on gift cards may apply. See Apple Store Gift Card terms and conditions for details.