pappy- i would still have that letter sent to interfolio. you have a database once you sign up, and you actually have to print a cover sheet for each teacher/dr/etc. who is writing you a letter. that cover sheet has something like a UPC on it, and interfolio scans the letter in when they get it. you give each letter a title, like "Dr. Jones, LOR, Orgo, DO schools", this way, when you pick to send you know which to send. especially helpful if you're applying DO/MD, and some of your letters are specific for osteopathic medicine. you can label one letter "Dr John - PCOM" and one "Dr John - General letter". its great, like someone said, if a school says they lost your letter, you dont have to bother the letter writer again.
If that whole thing sounds insane because my mind is kind of fried, sorry. but i swear to you, like the others, interfolio rocks! 🙂