Products with selling price is zero cannot be added to the cart, this issue occurs because the FastStore platform does not accept products with a price of the SKU is zero.

Simulation

Register a product with a SKU with zero price in the catalog; Trying to add to cart, you receive the following message: "The desired quantity for item {nameProduct} is note available"; And the cart remains empty;

Workaround

To enable it to be added to the cart, configure a 100% discount price for the product, restricting the promotion depending on the shipping method selected, so that the promotion is only granted once shipping has been selected.