Did you have the mobile data enabled?As this sounds like background data . Apps running in the background can occasionally make use of the mobile data.
It is best to switch off the data option under the device settings when the mobile data is not in use or when it comes to exchange your airtime credit for a goodybag.
I suggest you contact a giffgaff agent to see if they are able to refund you the 17p required to exchange the airtime credit for the £10 goodybag.
To contact an giffgaff agent use this https://giffgaff.com/support/questions