The way you buy the BB add-on has changed. You now select goodybag + add-on in one click.
If the goodybag you've just bought has gone live, there's nothing you can do until you buy the next one.
If it's queued, go here to cancel it: https://giffgaff.com/support/ask/cancel_queued_goodybag
When it's been cancelled (within 24 hours), go here to buy the goodybag with bb add-on