Hi, Ian -
My company is the official Avalara partner and wrote the certified AvaTax Connector for HikaShop. It works quite well but yes, there are some important things to know about how it works before deciding.
As part of our partnership with Avalara and HikaShop, we provide free technical support for the AvaTax Connector extension. I'll be happy to chat with you for a few minutes by phone to answer any questions. We're a USA company and are very familiar with USA tax issues, particularly the pitfalls of relying upon zip codes for tax rates (which is not correct).
Our phone number is easily found at the top of our website,
PolishedGeek.com
Looking forward to hearing from you,
~ Deb