State sales taxes used to be paid only on goods purchased within the state. Some years back, some states banded together and complained they were losing revenue to internet purchases outside their states, then shipped into them. In "the good old days" most people bought most stuff at physical local stores, paying local and state sales taxes as they did. Few people drove out of state to buy many things, and even fewer still ordered products through the mail. But, with the internet, significant numbers of purchases of significant items were happening. Not only were physical shopping places losing business, but the state and local governments were losing sizeable chunks of revenue.

A law got passed that certain large online businesses (biggest example--and the biggest target of the law--was ebay) had to start collecting all state taxes regardless of which state the item was physically bought in. I'm in Massachusetts. If I ordered an item from Tennessee, that Tennessee business had to collect Massachusetts state sales tax from me, and remit it to the state on a regular schedule, if it "qualified" to do so. This only applies to retailers of a certain size/volume which is all spelled out in the law. Shapeways was big enough. Every order, they added my state's sales tax as soon as I put in where I was from, though they were based in New York. 3D-Wild isn't required to under this law. They only charged shipping. "Too small" business volume-wise, and they're in Pennsylvania. If they'd been in Massachusetts, their software would have hit me with the state's tax as soon as I identified as a resident. That part has not changed. Order from an in-state company, you pay state taxes.