@troyroy there can be a couple of reasons for this. One being that you had left your data on and background apps running accessing your data which is charged to your credit if no goodybag in place.
Second is that occasionally there is the 1perfect glitch which is a system error and a message to an agent resolves this.
But moving forward to avoid both these happening again in future when wishing to buy a goodybag, you can now purchase goodybag's without the need to top up and then use this credit to buy a goodybag.
A straight goodybag purchase will avoid the issue you have experienced.