Subscribe Pro supports free (zero-dollar) orders using the default Magento 'onepage' checkout and the Subscribe Pro payment method without any customization. Other checkout extensions may not support this out of the box.
Note that all orders that should result in the creation of a subscription must be placed using the Subscribe Pro payment method, since a credit card must be tied to the subscription order.
The following screenshots illustrate a free order being placed on Magento to create a subscription in Subscribe Pro. In this example, the product is configured with a price of $0.00 and the Free Shipping shipping method is enabled for orders with a minimum of $0.00 as a subtotal.
Subscriptions created in Subscribe Pro require that a credit card be attached. In the above example, even though the order total was $0.00, the Subscribe Pro payment method was still available during checkout. In some cases, however, such as when applying a gift card or rewards points that cover the entire order, the checkout may say that no payment method is required, and when the user submits their order, the subscription will not be created because no credit card could be given.