So I was talking to my post office about hand cancelling my invitations. They told me they dont do it themselves but I could use the stamp to do it. BUT then they said that it really didn't matter. That all mail (envelopes) regardless of how it is stamped goes through a machine. They said long ago hand cancelled envelopes were handled differently but that today that is no longer the case. Everything is electronic and all mail must be counted. Soooo basically they said they give hand cancelling stamps to us brides because we are convinced that is the way to do it and they are trying to humor us, keep the line moving, and get us the heck out of their hair. So it may not be worth running all over town to find a PO you can hand cancel your invites.