This is exactly what I was looking for! My "sent" folder was getting so bloated with inane sent messages like "Thanks, see you at 6" that I don't need to keep. I was getting tired of wasting time trawling through my sent folder to delete unimportant messages every few weeks This add-on enabled me to set "sent" folder as default and "No copy" as implying "copy to trash" in my account setting. Now when I hit "send" on a message, the drop down asks me if I want to copy the sent message to "sent" folder or "no copy" (trash). Now my sent folder only contains messages that I need to keep for reference.
This review is for a previous version of the add-on (1.1.2).