Button in Klaviyo to add multiple items to cart in Shopify?

Hey @Chris-J 

Welcome to the community and thanks for asking for help!

Generally, we have the best of luck in using a combination of template tags, a for loop, and an if-else loop. Most recently, I have had the below logic work for a customer who was trying to repopulate the entirety of a customer's order, down to the exact same quantity of an item, in their Flow email:
 

{{ organization.url }}/cart/update?{% for item in event.extra.line_items %}updates[{{ item.variant_id }}]={{ item.quantity|floatformat:0 }}{% if not forloop.last %}&{% endif %}{% endfor %}

 
The reason why this works is largely thanks to how Shopify allows us to reconstruct orders in the cart page. We can begin a for loop that continues until the final item in the order (i.e. {% if not forloop.last %}), where each loop includes the item's variant ID and quantity, which is added to the cart in the redirect. 

Hope this helps!