Clo,
This is 'Franked' mail, meaning that they paid NO postage whatsoever. All they have to do is place a facsimile of their signature on the outside of the envelope and the Postal Service is required, by law, to treat it as if it were a First Class letter yet they get NOT a single dime for delivering any of it. The 'cost' sited in the study actually represents the money that the Postal Service LOST in postage stamp revenue versus if the Congressman's office had been forced to go out and buy stamps or at least pay for a postage meter.
I wonder if UPS or FEDEX will be as willing to deliver 'Franked' mail in the future if and when the Republicans manage to force the Postal Service out of business? I suspect that they will demand, and get from their Republican lackeys, a tax-payer paid subsidy to deliver the 'free' mail from your congressman and I'll bet that the rate charged will be way more than what it costs to deliver a First Class letter.

OCU