You can keep top-up £15 everytime,when you bought £12 goodybag,you will get £3 credit left on your account,and 4 times later,you can buy a £12 goodybag without a top-up
or you could alternate between £15 and £10 top-ups every other month.
not everyone has a debit card! its a stupid amount to pay
Most people could get one though.
Do you use more than 1GB of data per month? If not, just stick with the £10 goodybag. Or top up £15 one month and use the £3 left over towards next month's goodybag.