Why can't I return the sim card?

    Hello, I bought a sim card for my iPhone on Monday for like $15.9. However i found another un-activated sim card at home so I decided to return the new-bought sim card to the store. I visited the store yesterday. However, i was told that once the sim card was sold t-mobile would not accept a return anymore. I know if I buy something online t-mobile will send a return form with the item in case I want to return, but I did not expect that I could not return something I bought in store to the store?! If so who will buy in the store any more?

    The store is on 200 E Washington St, Ann Arbor, MI 48104.

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