I'm importing furniture from Europe via container. There are several variables to consider. Tax rate, shipping rate, shipping insurance, sales markup, initial cost of the stock, cost of trucks and warehouses to move and store things, cost of plane tickets when I have to go there in person, currency exchange. I had an elaborate spreadsheet that got more and more cumbersome until I was just confusing myself, so I wrote a simple one-page calculator to sort the problem out methodically. Just javascript and jquery, looks like shit on mobile, and it's still pretty crude, but it gives me a back-of-envelope calculation that's enough for my purposes.
http://rocketships.ca/srs/shipping/