How can we improve WHMCS?

In the United States, and presumably other countries, some areas have passed laws where web hosting services are taxable but appreciate certain benefits such as tax reductions or caps. For instance, in Texas, the taxable amount is only 80% of the value of the web hosting service. So if a host sold a VPS for $10.00, only $8.00 is taxable. Unfortunately, WHMCS offers no way to set a taxable percentage of 80%, so this often results in overcharging customers tax or providing unclear and inaccurate tax percentages to customers.

Given that there are many startups in states where these types of exemptions apply, it would be helpful to be able to add a "taxable value" field to products, or to add a percentage of value that is taxable to tax rules.