Shopify Scripts – Free product with purchase of Item. Also requires a Liquid integration.

add-free-product.liquid

//
{% assign found = false %}
{% for finder in product.collections %}
{% if finder.handle == 'name' %}
{% assign found = true %}
{% endif %}
{% endfor %}

{% if found == false %}
<ul>
{% for product in collections.knitwear.products %}
{% if product.type == "type or tags"  %}
{% for variant in product.variants %}
<li><input type="checkbox" name="id[]" value="{{ variant.id }}" />
<a href="{{ product.url }}" target="_blank" title="More details">
{{ product.title }} - Valued at {{ product.price | money }} 
</a>
</li>
{% endfor %}
{% endif %}
{% endfor %}
</ul>
{% endif %}

free-beanie.rb

cart = Output.cart = Input.cart

jacket_count = cart.line_items.select do |l|
  l.variant.product.product_type == "Jacket"
end.reduce(0) { |a, l| a + l.quantity }

beanies = cart.line_items.select { |l| l.variant.product.product_type == "Beanie" }
beanies.each do |l|
  break if jacket_count == 0
  if jacket_count < l.quantity
    split = l.split(take: jacket_count)
    split.change_line_price(Money.zero, message: "Free Beanie")
    cart.line_items << split
    break
  else
    l.change_line_price(Money.zero, message: "Free Beanie")
    jacket_count -= l.quantity
  end
end