I hope it's okay to post this here, but we just tried using this with our partner for sending audio files back and forth, and it was more convenient than Dropbox or Google Drive: Rymdport. It uses a protocol called "Magic Wormhole" to let two random computers that do not have their own Internet IP addresses send files back and forth.

The interface is pretty janky (for some reason, uppercase letters and lowercase letters are sorted in different parts of the file select window?) but it's free, there's versions for Windows, Linux, and MacOS, and we didn't have to deal with file size limits or storage limits or anything.