You may want to read this first before offering your products to USA based customers
http://www.cbp.gov/linkhandler/cgov/newsroom/publications/trade/iius.ctt/iius.pdf . It a 211 page OVERVIEW of what must be done by an international seller prior to the item arriving at the US customs clearinghouse, rather than what the stateside customer is responsible for. Any seller that is willing to sell internationally should look up sites like this one from the countries they wish to sell to because there are many odd-n-ends law and regulations that maybe missed by your national postal system about what can really be sent to another country.
What herflyinghorses said is true, under the CPSIA an US customer is responsible to make SURE testing is done, but you may still need to do the testing before the item is sent. There are people and government agencies just interpeting this law in their own view, & some maybe custom's officals that read the law as being it must be done prior to shipping to the USA and the shipment must include all appropriate paperwork to pass though the clearinghouse. So that translates that the customer must pay you (additionally) to get the product tested to the USA standard (or show exemption from testing) & you must fill out a GCC showing compilancy, it does not exempt you from doing the actual testing or paperwork involved it just means your not directly financially responsible to pay for the testing (or time spent proving exempt status) and your extra time to fill out paperwork not required by your own government.
It's a bit like me sending costume jewelry to Italy, a few custom's officals there read the jewelry clause of their importing law to mean that no jewelry at all can be sent there without a special import license and import/export expatriation agent handling th shipment. While I've had people here read the clause to mean that the law only refers to fine jewelry that contains semi-precious (or better) gems set in sterling silver, gold, or platinum. So if I ever offer my products to someone from Italy I'm planning on adding that additional set of fees for the license and agent to the shipping amount. Since the money needs to be paid prior shipping to Italy and the paperwork needs to be included with the package (like with the CPSIA), rather than a tarfit or import fee charge by the recieving country's government or the shipping company to be paid by the customer to the delivery person, in my mind I would need to add to the listing the reason that that the shipping there is 5x more expensive than shipping elsewhere in Europe.