Custom WHMCS plugin to disable sending of $0 (free) invoices / payment confirmations

Hi guys! For any of you that use WHMCS, you probably know that it has no option to disable sending of $0 (free) invoices and payment confirmations. What happens is that any products/services that are offered for free (for example, say a customer has a paid VPS and is allowed to host as many sites as will fit on the VPS. I don't charge them for individual hosting per domain, but if the customer has 20 domains, they receive many useless invoice/payment notices per month that say "Paid $0". Very annoying. So what I'd like the provider do is create a WHMCS plugin that will effectively disable the sending of $0 invoices / payment confirmations (because it frequently confuses customers). Optionally, I would like to have the possibility to override this setting on a per customer basis by adding a custom checkbox to the customer's profile. I'm open to suggestions on the best way to proceed with this functionality, but if it works well, then I may decide to encrypt / license the plugin for resale. The plugin should be coded properly, using WHMCS best practices, and the provider should consider that future support/customization may be necessary to prevent the plugin from breaking as new WHMCS versions are released. Questions? Just ask! Thanks!