You are right to be worried..
There have been several "mail horror stories" concerning this series of RRs.

I believe our RRs look like "suspicious packages" and I believe it stems from the use of postage stamps, for two reasons:

1. Many postal bombs in the past have had many stamps on them.

2. Postal clerks make a mistake (it happened twice during these RRs) because the complete postage is already on the package, they forget to scan the "confirmation slip" which puts the confirmation number in the PO computer.
When handlers get the package with a bunch of stamps and a confirmation number that is not in the computer, they get nervous.

In the future, I will no longer ask participants to use stamps.