AndIt’s a possibility that some of the airtime credit was used by mobile data as soon as you topped-up. It's always best to disable 'mobile data' whilst you're exchanging the airtime credit for a goodybag Or better still buy the goodybag direct using the credit/debit or Paypal method.
You can try contacting a giffgaff agent regarding this however, it will be at their discretion to process a refund for the £1 so you can exchange the airtime credit for £25 goodybag.
• This is when background apps are running and use a small amount of your credit , iam sure if you ask a agent they will as a good will gesture refund the £1 ( at there discretion) to enable you to buy your goodybag
•To prevent this from happening in the future try to top up or buy goodybags whilst on Wifi