Basically, they hire a middleman. Procter & Gamble, for instance, realized that although Walmart is its single largest customer, System D outposts [off-the-books, self-reliance economies], when you total them up, actually account for more business. So Procter & Gamble decided to get its products into those stores.
In each country, P&G hires a local distributor—sometimes several layers of local distributors—to get the product from a legal, formal, tax-paying company to a company willing to deal with unlicensed vendors who don’t pay taxes.
That’s how Procter & Gamble gets Downy fabric softener, Tide laundry detergent, and all manner of other goods into the squatter communities of the developing world. Today, in aggregate, these markets make up the largest percentage of the company’s sales worldwide.
— Robert Neuwirth in an interview with WIRED’s Robert Capps, Why Black Market Entrepreneurs Matter to the World Economy