Linking to a product as described in the documention is what adds a product to the cart. Blesta's services have more options than I think WooCommerce products allow for, so there is additional configuration typically necessary for products, (e.g. config options, module options, addons, etc.) and what appears and where also depends on the order form used, but you can preselect most of those options and have them set in the cart per a link to the product, then the customer can fill out anything else and proceed with checkout.