Custom fields allow you to collect additional information from or about your clients, from birthdates to website URLs to customizing a service's password. WHMCS allows you to set up an unlimited number of these fields, and you can set them as either admin-only for private use or public.

For more information and instructions, see Custom Fields.

Configurable Options allow you to give your clients variables which alter the price of that product. You can define them in groups that you can then apply to one or more products.

For more information and instructions, see Configurable Options.

Product Addons let you offer additional items that the client can select. These are billed separately from the product.

For more information and instructions, see Product Addons.

In WHMCS, your clients can upgrade or downgrade their products and packages directly in the Client Area. WHMCS can fully automate these upgrades and downgrades for many provisioning modules.

For more information and instructions, see Automated Upgrades and Downgrades.