volusion actually does everything you need...I've seen it set up with GST and PST and it's flexible enough to account for where you're shipping to / from (based on how the site sells). it basically has a table you can point out every tax point based on as much detail or as little as you need. you just have to set this up in the settings not "change the software"...
plus the nice thing is it is all hosted, setup for you, and not very expensive....just my opinion though
