Amy stopped the mailman and asked him if he delivered the object of mystery:

Amy:

Talked to the mail man. He remembers delivering out mail yesterday, but he didn't deliver that. He said it was very weird and that the stamp would have been canceled not just crossed out like that. He said someone must be playing a trick on us.

I am happy our mailman can remember yesterday. But she quickly followed with this!

He remembers delivering OUR mail yesterday, not "OUT mail"

So it seems to be a local job and not a plot against the Postal Service. This narrows the suspects considerably. The bad thing is, I really don't know many people that live around here and I know none that are this diabolical. But I'm hot on their trail. I bet the perp is scared now.

Can you feel the noose tightening? CAN YOU!?!