these additional extras are charged in xbox live points which is pre-paid credit bought pretty much like a top-up card on a mobile phone.
2100 points is around 20 GBP.
Personally I dont have my bank details on XBOX because the less they know the better. I just use the pre-paid XBL 12 month membership card, which again is pre-paid and used like a top-up card.
Maybe someone else can shed some light into how buying stuff on your bankcard works? I'm sure you'd need to enter the email address and password associated with your xbox live account?