Can I use my balance of £5 to add towards the cost of a goody bag
You cannot use the £5 as part payment for a goodybag I'm afraid.
You could add credit of £10 then use the £15 to purchase a £15 goodybag
As the minimum top up is £10 then you will be able to buy the £5, £10 or £15 goodybag, so sorry about that, the reason you cant do what you are asking is that you cant just add £5 airtime credit.
(Be carefull not to turn on your mobile data otherwise some usage can reduce your airtime credit before you buy the goodybag.)
i hope this helps.