Please read this briefing by the US trade commission https://www.usitc.gov/publications/332/executive_briefings/e...
"The illicit trade follows the standard e-commerce model of small-scale shipments and B2C and B2B distribution channels. The trade is SME-focused, with small-scale Chinese suppliers primarily exporting to individuals and small independent U.S. criminal networks.China was the source of 97 percent of inbound shipments of high-purity fentanyl during 2016 and 2017"
"Given the high purity of Chinese fentanyl, most illicit imports are less than 700 grams (1.5 lbs.) per shipment. "