If you inadvertantly buy chametz on Passover, you must burn it.
If chametz was brought to you, such as by mail delivery service, DON'T accept it. Consider it as ownerless/hefker. You may not bring it into your house or yard. If it is still there after Passover has ended, you may take it for yourself.
Note If  you inadvertantly bought kitniyot during Passover, just put it away until Passover has ended and then you may eat it.