good morning @yasin010869 this is not nice when this happens and your balance goes down to £24.99
sometimes giffgaff accounts are subject to a glitch where 1p or 2p is taken from accounts. The giffgaff agents will be able to sort this for you, just explain what has happened to you.
To get in touch with an agent click https://giffgaff.com/support/questions
In future you could just link a debit/credit card to you account and buy a goodybag directly here.
It's always better to disable mobile date prior to topping up your phone. This is because the moment that credit is added to your phone, apps will automatically begin working in the background using your data .
Hope this helps you