This is a neat product and discount idea! Do you know which free item you wanted to add to your customer’s cart?
I think you could create an automatic discount. Automatic discounts help you add a discount automatically to the cart and the discount code when all eligible items are added to the customer’s cart. You can add a fixed amount, percentage amount, or make a Buy X, Get Y discount.
I think a Buy X, Get Y discount would suit your needs here. Buy X, Get Y discounts require the customer to add a specific amount of products for a discount, and you can choose the number of items a customer needs to buy from your products or collections, and you can choose what the customer gets from products and collections, too.