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).
Not additional stops - different FLOWS. This stuff had to be routed to specific collection locations. Incorrectly addressed letters (imagine that from the govt - it's hard, I know) had to be resolved and delivered if at all possible. Drastically increased manual (expensive) handling etc. etc.