I bought a USB memory stick, which I had left in my pant pocket and my wife accidenlty put those pants in the washing machine. Can I return/exchange it since the store has a 30-day return policy?

You can return the memory stick only according to the terms and conditions of the return policy.

If the policy is that the item may be returned if you do not damage the item, then it will not be permissible for you to return the memory stick as you will be violating the terms and conditions of the return policy.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai