"Interstate commerce" means across state borders, not the particular route it travels. USPS to a different state is sufficient, so long as the sale is not just one sham sale for the purpose of achieving "use" but is rather in the ordinary stream of commerce. I'd say sell a bunch of them through ebay.com -- keep doing that until you get a reasonable number of out-of-state buyers. Even better is a lot of product to a retailer in a different state.
Just as a side note, the reason for the requirement of use in "interstate commerce" is that the Constitution gives Congress the right to regulate interstate commerce. Wholly intrastate commerce is, theoretically at least, beyond the reach of Congress. Caselaw makes it clear that Congress can also regulate intrastate commerce to the extent it affects interstate commerce, and that just about any intrastate commerce has an effect on interstate commerce.