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 can be impractical if the folder is large, data is limited, or network is slow, and is especially undesirable if you only need a fraction of a large number of shared files. The shared folder should offer a selection of files for download, acting similarly to a network drive, rather than mirror all shared data.
Peer to peer only. Must not transfer any file/folder data on a third party, no matter if it's encrypted or not. (Third party can be used for connection setup and/or metadata) This rules out cloud services and file hosting sites. This can make the whole process much faster since it's a one step transfer, instead of upload to server then download to client. This also allows it to take advantage of fast LAN networks.
Low configuration and low privilege requirements I would highly prefer a quick & dirty solution over something that requires lots of setup/configuration. Also, 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). So the solution must not require manual network configuration, admin privileges, or other changes that users typically do not have access to. Many users will also not be technically inclined, so 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.
I'm looking mainly for Windows, as that is the most common, but welcome solutions for any OS including mobile. 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 crucial:
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.