This was previously a bit difficult due to the nature of our pricing calculations; all of our calculations came from a stored procedure called GetCartSubTotalAndTax. Currently to make such an integration you'll need source code and you'll need to make a few changes to TaxTotal in the ShoppingCart class (there are other places as well if you are using VAT) and probably the stored procedure as well.
We're currently implementing a plug-in interface that will allow you to make such integrations much more efficiently, which is part of the integration previosly mentioned and of the main features recently demonstrated at our developers conference. Keep an eye out soon for this
<a href="http://www.aspdotnetstorefront.com">Shopping Cart Software</a>