#cart {
    position: fixed;
    top: 0;
    right: -500px; /* hidden outside the screen */
    width: 500px;
    height: 100vh;
    background: #fff;
    box-shadow: -2px 0 8px rgba(0,0,0,0.2);

    transition: right 0.3s ease;
    padding: 20px;
    z-index: 999;
  }

  #cart.open {
    right: 0; /* slide in */
  }