Shopify Pre-Order Matrix

pre-order-form.snippet

{% assign preorders = 0 %}     
{% for item in cart.items %}            
  {% if item.product.tags contains 'coming-soon' %}       
    {% capture preorders %}{{ preorders | plus: 1 }}{% endcapture %} 
  {% endif %}
{% endfor %}

{% if cart.item_count == 0 %}
  {% if product.tags contains 'coming-soon' %}
      <input type="submit" name="add" id="add" value="Pre-order Now" class="purchase button">
      <input type="hidden" name="return_to" value="/checkout" />
  {% else %}
    <input type="submit" name="add" id="add" value="Add to cart" class="purchase button">
  {% endif %}
{% else %}
  {% if preorders == 0 %}
    {% if product.tags contains 'coming-soon' %}
      <p class="warn">You have non pre-order items in your cart. Pre-order items must be ordered separately. Please <a href="/checkout">click here</a> to complete your order for the items that are already in your cart.</p>
      <input type="submit" disabled="disabled" name="add" id="add" {% if product.tags contains 'coming-soon' %}value="Pre-order Now"{% else %}value="Buy now"{% endif %} class="purchase button">                  
    {% else %}
      <input type="submit" name="add" id="add" value="Add to cart" class="purchase button">
    {% endif %}
  {% else %}
    {% if product.tags contains 'coming-soon' %}
      <input type="submit" name="add" id="add" value="Pre-order Now" class="purchase button">
      <input type="hidden" name="return_to" value="/checkout" />
    {% else %}
      <p class="warn">You have pre-order items in your cart. Pre-order items must be ordered separately. Please <a href="/checkout">click here</a> to complete your order for the items that are already in your cart.</p>
      <input type="submit" disabled="disabled" name="add" id="add" {% if product.tags contains 'coming-soon' %}value="Pre-order Now"{% else %}value="Buy now"{% endif %} class="purchase button">
    {% endif %}
  {% endif %}
{% endif %}

This snippet stops people buying pre-order and non-pre-order items in the same checkout through a simple unobtrusive set of restrictions and messages