Taxes are managed using a separate standard Joomla plugin, geared to work with the SimpleCaddy data. This tax plugin is made for the North American pricing system. That means that all prices are shown without taxes and taxes are added at the last moment before payment. The reason for this? Many states in the US and in Canada have different tax rates and charge taxes using different rules.
Taxes are calculated using the Custom field (Checkout field) you have designated to hold the billing address of your clients.
The Tax is calculated by adding to the final cart display, just before the payment gateway page.