If i understand you correctly you want to buy a goodybag with your £25 credit that is already on your account ?
If so then just make sure to switch off recurring goodybags first as this will prevent you using your crefit otherwise. You can do that by following the first link added below. (Don't forget to press Save after altering the recurring setting)http://giffgaff.com/buy
Pick your goodybag
Press 'I only need a goodybag' on the next page'
Continue then press Exchange Airtime Credit to pay for it with your balance.
The goodybag will immediately activate on your account
p.s. don't worry about this thread being in the wrong part of the forum, i'll inform the admin and they'll sort that