Depending on your needs, you may or may not use Extra Cart Button(s) in conjunction with "Free Sample" functionality of the plugin. Still, we recommend review the related settings.These settings are general preferences. It means that a particular Extra Cart Button may or may not be configured to add "free" (i.e., with price equal to "0") product to the cart. But if any such button is meant to do that, you'd better enable this functionality here, just as seen on the screenshot. Also it is possible to limit the number of "free" products per order. Use the second setting for this. Leaving it empty or setting to "0" allows unlimited number of "free" products per cart.
👆 Generally, it is not possible to add "free" product in WooCommerce. Neither it is possible for a product configured with Uni CPO as "add to cart" button will be blocked if the price calculated equals to zero. Extra Cart Button is the only way to add "free" product to the cart, so it can be used if you are willing offer free samples of your products to your customers. 😎
Pleas, proceed to this page to read more about configuring Extra Cart Button option.