In that case, Shopify could work for you as you just need a US shopfront. It was programmed by Canadians, so they have it working ok for a jurisdiction where sales tax is added on to the price. It's just for jurisdictions where the advertised price has to include the tax where it fails.
We're an Aussie clothing retailer, we sell in Aust and now in NZ. But the problem is that Shopify can't take off the Aust GST on sales to NZ or elsewhere. Very frustrating.
We're looking at migration options, BigCommerce included, but not yet familiar enough with it yet.