The only thing that concerns me is the perps that did this must feel like they will be safe from being caught by the FBI, Secret Service, Batfe, etc. I say perps because one person could not have been all over the country in one day. They must feel like they are safe from the FBI. That's scary.
The reason the stamps were not cancelled is that it never got past the Postal Sorting Center.
Anything going to someone with Secret Service protection has their mail diverted to a special facility that can screen for just these things.
Hillary and Barry are, so why shouldn't they?