Как не подбирать max-height?

* {
  box-sizing: border-box;
}

.wrap {
  display: flex;
  flex-wrap: wrap;
  margin: 0 -12px;
}

.item {
  position: relative;
  width: 306px;
  height: 415px;
  margin: 0 12px;
  margin-bottom: 24px;
}

.item .inner {
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  max-height: 100%;
  border: 1px solid #E9E8F4;
  border-radius: 15px;
  padding: 15px;
  padding-bottom: 5px;
  background-color: #fff;
  overflow: hidden;
  transition: max-height .5s, box-shadow .5s;
}

.item img {
  width: 100%;
  height: auto;
}

.item:hover {
  z-index: 2;
}

.item:hover .inner {
  max-height: 120%;
  box-shadow: 0 0 30px -5px rgba(0,0,0,.2);
}

<div class="wrap">
  <div class="item">
    <div class="inner">
      <img src="https://cdn.shopify.com/s/files/1/2570/9456/products/My-Name-Label_Personalised-School-Bag-Tags.png?v=1534927228" alt="">
      <h4>Рюкзак 3000</h4>
      <div class="info">
        <p>скрытый блок</p>
         <p>скрытый блок</p>
      </div>
    </div>
  </div>
  
  <div class="item">
    <div class="inner">
      <img src="https://cdn.shopify.com/s/files/1/2570/9456/products/My-Name-Label_Personalised-School-Bag-Tags.png?v=1534927228" alt="">
      <h4>Рюкзак 3000</h4>
      <div class="info">
        <p>скрытый блок</p>
      </div>
    </div>
  </div>
  
  <div class="item">
    <div class="inner">
      <img src="https://cdn.shopify.com/s/files/1/2570/9456/products/My-Name-Label_Personalised-School-Bag-Tags.png?v=1534927228" alt="">
      <h4>Рюкзак 3000</h4>
      <div class="info">
        <p>скрытый блок</p>
      </div>
    </div>
  </div>
  
  <div class="item">
    <div class="inner">
      <img src="https://cdn.shopify.com/s/files/1/2570/9456/products/My-Name-Label_Personalised-School-Bag-Tags.png?v=1534927228" alt="">
      <h4>Рюкзак 3000</h4>
      <div class="info">
        <p>скрытый блок</p>
      </div>
    </div>
  </div>
  
  <div class="item">
    <div class="inner">
      <img src="https://cdn.shopify.com/s/files/1/2570/9456/products/My-Name-Label_Personalised-School-Bag-Tags.png?v=1534927228" alt="">
      <h4>Рюкзак 3000</h4>
      <div class="info">
        <p>скрытый блок</p>
      </div>
    </div>
  </div>
</div>