A huge component of the USPSs budget deficit is its funding of retiree health benefits. When the USPS was on the plus side it was arranged that the payments be accelerated. It is roughly the equivalent of paying off your 30 year mortgage in 10 years. The USPS has repeatedly asked congress to allow a return to the original schedule - no dice so far. Through facility reductions, employee attrition and revisions to process flows the USPS has logged substantial cost reductions every year to respond to declining volume.
As far as census mailings being used to add funding to the USPS - I really doubt it. The odd flow of this mail and the special effort that the USPS had to apply to deliver this stuff made it more costly to handle than the average letter. In all likely-hood it cost the USPS more than what was probably paid.
Simply by having the census mailings entered as Standard Mail USPS should have been able to break up the load so that it didn't force any more delivery stops on any given day than is normally required on any given day.
Leastwise that's how it was done in 2000, and 1990, and 1980, and 1970.
No reason whatsoever to do it differently in 2010 (with regard to delivery).