Taking £29 charges off my account

I recieved a £10 discount voucher from Allaboutprizez.com offering me discount off of my next Amazon order. Naturally I thought this was to do with Amazon and the fact that I order from them reguarly. I was surprised to later find they had taken £29 from my debit card, which then put me in fees of my overdraft! I would like to know who to challenge as I did not agree to any annual or monthly membership or subscription, so see no reason why they should take money directly from my account when I thought my details were secure with Amazon.