Herbert Heaton, in his old but well-researched (and wryly humorous) article "Non-Importation, 1806-1812," observed that even during the Embargo, when American trade with foreign nations was against the law, an enormous quantity of British goods entered the United States - 3.9 million pounds sterling worth in 1808 alone. This was half as much merchandise as Britain exported to the U.S. in 1807, but the figure should have been zero. Heaton suspects that most of these goods were carried in American ships that had cleared port and headed across the Atlantic before the Embargo took effect. Some may even have left after that date: at least one port inspector, in New Orleans, didn't enforce the Embargo for six weeks after he learned of it. (Journal of Economic History 1 [November 1941]: 183, 189-191.)
To the extent that most American merchants complied with the Embargo, they did so because they could exploit small legal loopholes like these - or because they expected it to last no more than a year. At the end of 1808, as the anniversary of the Embargo Act approached, merchant ships traveled up and down the Atlantic seaboard, loading American produce (wheat, tobacco, etc.) in the expectation that they would be able to put to sea in the new year. When Congress instead passed a new law tightening the Embargo (Jan. 1809), many captains set sail in defiance of the law, knowing that the customs service and the navy lacked sufficient vessels to stop them. At least 100 Embargo-breakers reached Britain and Europe by the time Congress finally killed the "Ograbme" in March 1809.