We need to pay US sales tax only for sales by customers located in California. Everywhere else it is 0% (other states and countries). We have it set up in Ebay correctly, but in Linn programs we seem to have to fill in the tax and it applies everywhere as part of the sale price (whereas in the US it is added to the sales price if customers are in California).

This is a MUST have for American market!

To do this correctly, you really need the ability to import an entire table of zip codes for the state with different sales tax rates, as many states like Kansas require destination sales tax based on the county it is delivered to.

Without the ability to calculate taxes correctly, Linnworks has significantly limited benefit to users in the USA.

Absolutely gotta have this.

To make things easier on the County data issue, you can convert cities to counties using the SBA's API. Here's the API, but keep in mind that some cities span multiple counties, so always pull only the first listing unless you know of a better way. This is how I built my module for OpenCart. http://api.sba.gov/geodata/all_links_for_city_of/Strongsville/OH

Check out Dallas TX for an example of a multiple county city.