It's the age old problem of transferring files between two computers over the internet, but with a few restrictions:
Must not sync. Syncing requires uploading/downloading all files in folder. This is impractical if the folder is large, data is limited, or network is slow, which is especially problematic if you only need a fraction of a large number of shared files. The shared folder should act similarly to a network drive.
Peer to peer only. Must not store any file/folder data on a third party, no matter if it's encrypted or not. This rules out cloud services and file hosting sites. This is great for fast LAN transfers, but must also work across the internet.
Does not require technical expertise or admin privileges. Since this is mostly a temporary, spur of the moment thing, I would highly prefer a quick & dirty solution over something that requires lots of setup/configuration. Both server and client are frequently behind a router, and configuring port forwarding is not only a hassle, but may be impossible on networks you do not control (e.g. mobile LTE, public WiFi, or school/work network). And since many users will be not technically inclined, complex configurations are not practical, even if it was possible. It should ideally be simple enough to guide a normal user through setup in 5 min over the phone.
Mainly for Windows, as that is the most common, but solutions for other OSs are also welcome. Cross platform solutions are also a huge plus, but not required.
Paid options can be considered, but since it's for individuals, it would have to be pretty cheap to justify the cost, and also rules out the many enterprise solutions that exist.
I have found many programs that come very close, but are always missing something.
Syncthing: Everything is great, except that it requires syncing.
FTP/SFTP: Great solution, except setting up the networking correctly is nontrivial.
FilePizza: Great solution, except it only transfers individual files.