I work at the "post office" window of a university. We are employees of the University, not the US Postal Service, and we are not an official post office (not even a contract station), but we function much like one. We can accept packages, compute postage rates, sell stamps, etc.--mostly for students and staff, but occasionally for the general public.

I can testify that some people, particularly foreign students, particularly Japanese students, do carry $100 bills around with them. I suppose it's a cultural difference. Things are more expensive in Japan, and robberies are less common. It's always a problem when somebody tries to give me a $100 bill. Sometimes I don't take in that much cash in a day, and I only start out with $60 in the cash register. The food service here doesn't accept any bills bigger than $20, either.

Then there are other student who don't carry enough change to buy a stamp.

I almost never have big bills myself, because nearly all the cash I spend comes from an ATM, and I don't know any ATM that dispenses bills bigger than $20. The only ATM on campus dispenses only $10 bills.

The biggest "security" problem we have is a rule that the USPS imposes on us: we are supposed to ask to see an official university picture ID of anyone who tries to mail a package heavier than 1 pound. If anyone drops off a package with us without showing an ID, we are supposed to label it "TARGET MAIL" which means the USPS will subject it to closer scrutiny, and they might return it to the sender. I don't know what criteria they use to decide this, or if they are consistent.

This rule is older than Homeland Security. I am told it goes back to the days when Ted Kaczynski, the Unabomber, was sending bombs through the mail.

In the interest of good public relations, we would like to provide this service to the public (accepting heavy packages), but we can't.

Ironically, Federal Express and UPS don't have this rule, but they are usually more expensive than US Mail.