Unlike other eCommerce plugins, which has a lot of community development such as: hacks, plugins, add-ons, modules; cart66 has none. The reason being they do not fulfill any feature requests by the community.
They have an update once every 4 months or so, and it just says:
Not to mention its mostly to do with something trivial like a "," or "." or some special character, or again... "live rates" or whatever that nobody even uses.
You rarely see any
See for yourself: http://cart66.com/latest-cart66/
And keep in mind this is the PRO version.
The Lite version hasn't got any "*New" for over a year probably.
You will end up paying programmers for the very basic add-on features you want.