They subsidize it by charging a rate per piece that is less than a regular stamped mailing, but at a price level that still turns a profit.

It's market segmentation, full price service for those willing to pay it, and a cheaper option for business without enough margins to justify that.

Let me clarify my argument. Ostensibly, the goal of the USPS is to connect all of America with flat-rate first-class mail. The USPS charges as much to send a letter across the country as it does to send it to your apartment neighbor. That means that people in cities (or really, people in routes with high density first-class mail usage) subsidize people in lower-density areas. Let’s assume that we agree that this goal is reasonable, and thus that this subsidy of low-density customers by high-density customers is reasonable.

This means that mass-mailers are being subsidized by the USPS’ implementation of this goal of nationwide first-class mail, because presumably they are paying less than the actual cost of mailing their advertisements. I suppose we could go a step further, and say that high-density customers are in fact subsidizing mass-mailers, which I think most people would agree is not a reasonable subsidy.