Yes, you can mail a package at Diagon Alley. Also, at Hosgmeade as well, where you originally only was able to mail letters. It's called "Owl Post" at Diagon Alley. It's near what is considered the exit point of Diagon Alley. It's a tall building, purple, there are a bunch of holes and you can see owls in them.
Does anyone know if you can send a letter or package to the UK from here? A friend’s grandson will be visiting in October and I thought it would be fun for him to get some owl mail, maybe with with a few Universal goodies. I’ll be at UO last week of March.
Due to the uniqueness of that question, if someone doesn't respond, you will have to call ahead if you truly want an answer for that now rather than later, or just be willing to ask when you get there. I would imagine if you do, the price will be additional. However, the whole experience and price already differs depending on what kind of packaging, string you choose, size, (ect) of the package.