You can definitely spend your £15.02 credit balance on a £7.50 goodybag (and have £7.52 left for next time).
If it's not letting you do it online or on 43430 it is almost certainly because you have already given the system instructions on what to do for your next goodybag (EG you have already queued one, or you have turned recurr on and given that an instructon). You just need to turn recurr off or cancel the queued bag as you can't have 'two' different instructions for your next bag!
The important instructions on this page explains both and how to solve them. I suspect you have 'recurr my goodybag turned on and already asked it to buy the £10 next from your card? But it could be a queued bag. Check both if you aren't sure what you've previously done!