Comment Need US law and exempt the first $5K/state (Score 1) 507
If I have a small business in CA and sell less than $400/year retail, the state doesn't want to be bothered setting up to collect sales tax from me. Similarly, if I run a small candy store in Vermont and ship something to CA a few times a year, it isn't worth anyone's trouble to collect sales tax. That said, if I do sell within my state, I have to compete with businesses that pay no taxes.
Here's a reasonable solution....
1) If you ship less than $5K/year into a state and you aren't located there, don't worry about it.
2) If you ship more than $5K/year into a state, then you must check with that state (in a nationally standardized way) to find out if they have opted to tax incoming items. If they have, you have to pay the same tax rate as their in-state businesses, but you get to send it to the state along with a standardized (probably CSV) file indicating the zip-codes and totals you have shipped to.
3) If you ship from outside the US, you can either follow the procedure in (2) above, you can let ebay/paypal/Visa/MasterCard/Google handle that for you, or you can pay a standardized 10% fee by the same mechanisms that handle import duties now. Common Carriers (USPS, DHL, FedEx, UPS) would find it necessary to ensure that they either have confirmation that the duties have been paid or that they collect them from the shipper (just as they presumably do now if you try to ship something non-duty-free from abroad).