Create a product addon. Product addons let you offer additional items that the client can select when purchasing the parent product.
Enable cross-selling in WHMCS. You can select the locations in which recommendations appear and configure how they display to customers.
Enable taxes in an individual product's configuration. WHMCS will not add taxes to each of your products until you enable them.
Hide or retire products when you want to stop offering them for sale or only want to offer them through direct links (not in the store).
Link directly to the shopping cart, specific products, or product groups. You can find links in the product and product group settings.
Configure free trials for your products in WHMCS. Free trial accounts automatically terminate a set number of days after ordering.