Device is useless to me

Bought a phone in car hands-free device, we tried it and it did not stick even without the phone in the cradle, I was not able to return it in 7 days and when I did I was keen to take the more expensive device, but since it was not within the return policy which I understand but it says on the slip the device has a 6 months or year warranty , the person at the shop tested it and saw how it did not stick It is useless if it does not stick really , but she said there is nothing she can do sorry , no we cannot swop it or send it back to get it fixed nothing , I am now stuck with a device that is pretty useless to me, not happy about warranty and there is nothing I can do.