It did come at 9! The rest of my mail came before that though and I'm used to my RV items coming at the same time as all the rest of the mail (at like 6am, I believe).
Edit: It also skipped 6pm delivery time yesterday. I think I ordered before noon yesterday.
I've noticed that generally mail from special characters like Gulliver and Katie and occasionally villagers will arrive at 6, but orders from the nooklings and the campground tend to stick to 9 and 17
I think they actually require a day cycle? I think it has to do with how many items you can order per day, they need to be delivered before that number resets. That's just my experience so far