Need Marketpress API documentation

Hi -- I can develop a plugin to calculate destination based taxes for WA state (USA) but I need to see the action and filter hooks for the marketpress shopping cart. This is not the same as building a gateway plugin, so the inline documentation referred to in other threads is not quite sufficient. Can anyone at least let me know what action/filter hooks would be appropriate for changing the tax rate when the cart items or shipping address are updated? Thanks!