Great question! The checkout flow for the customer will change depending on if the item is free, has free shipping and if the customer’s address is in an eligible location.
If a customer is purchasing a free product that also has free shipping, this is what the final stage of their checkout looks like:
If the product is a physical item, the customer will be required to still enter their contact information to ensure they are within the eligible shipping limits of your business.
If there are other payment method buttons added to product pages /cart that lead to external services it’s best to make a test order through those buttons.
If using an app to manage free item(s) it’s best to make a test order to be sure of the flow that app creates such as if it collects an email address it may send a checkout link to that email address as a way of verification instead of sending them right to checkout.