This article applies to Classic Commerce. (Looking for Contextual Commerce documentation?)
If you set the price on a product to be $0.00, then when a customer goes to purchase only that product, our order page will display the message "Order total must not be zero." To allow customers to purchase a product for free, you must also create a free discount offer and then add a product condition, which will limit the free discount to apply only to a specific product. Note: FastSpring does not charge a fee on free orders.
Go to SpringBoard's Store Home » Promotions and Appearance » Offers. Click Create Discount.
Enter in a name for your discount that is used to for managerial purposes. Under Discount Types, select Free. Under Conditions, select the checkbox for Add Product Condition and select the product you want to make free. Click Next. Move to Active Status.