Mis sold & ripped me off

I purchased a good deal at a sainsbury store on promotion.knew it was to good to be true. Rep told me no other costs incurred. Wanted to speak to husband but pressured sold it to me. When I booked my sons birthday party they wanted to charge me another £10 per person. I said this was not what was agreed. Decided on a refund. As suggested by company sent special delivery royal mail £6.45 for the privilege and then got a call to say I was getting a refund minus £5 administration. No need to refer to to complain no email no number a person less dept conveniently. So lost £11.45 with for the luxury of guy mis selling to me. All rip off merchants. Don't be fooled with the 'great deal' it's all a way to make more money!!