Folks have been signing up for the project and I've been mailing out envelopes with stickers in them. It's been a few weeks now, and most of the first few batches have been reported delivered.
Early results are indeed starting to form some patterns.
First of all, most mailed envelopes reached their destination in 3-4 days regardless of how far it travels.
Secondly, some took considerably longer to reach their destination. Again, distance traveled did not seem to be a factor.
However, this is such a small sample size. More participants would be helpful.