Actually, the USPS solicits those because it makes more money on those than on regular mail. But since you claim that it's mandated by Congress to provide that service at below cost, I'm sure you can provide the reference to the appropriate law in the US Code or regulation in the Code of Federal Regulations.
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Of course, the legislation does not mention this in such words, but a piece of junk mail weighs as much or more than a 44 cent letter and often weighs more or takes up more space. I suggest YOU provide proof that the USPS makes more per item on junk mail and catalogs than than it does on a 44 cent letter.