Are you allowed to put an envelope in your personal mailbox that will be picked up on Shabbos or Yom Tov?
The Sanctity of Shabbos by Rabbi S. B. Cohen pg. 107 writes: "One is also permitted to place mail in his personal mailbox on Friday. (If one knows, however, that his mailman is Jewish, this is prohibited.)" Amongst his sources is אגרות משה או"ח ח"ג [סוף] סי' מ"ו.
I have never asked a posek, but I have done so since I do not specify a need for it to be picked up on Shabbos, and Sunday works just as well. The post office chooses their own time to pick up the mail (Its their preference to pick up on Saturday, not Sunday) so they are doing melacha on Shabbos for themselves.
Claiming that placing the mail there or raising the flag is similar to a command would mean I am not allowed to drop off my clothes at a launderer if I know for sure he's going to clean them on Shabbos, yet this restriction is not stipulated in the halachic sources.
I know of one rav who went into the post office on Friday to send an overnight letter (which would be a problem) and tried having a conversation with a confused postal worker about how he didn't want it delivered until Sunday!