i always serve my files through dropbox, either with a link or by sharing folders with regular customers. i have a bunch of clients who like to drop their files in their shared folder so it uploads to my studio overnight. works great for mixchecks, and dropbox has an internal mechanism for checking the integrity of files so no need for zipping anymore.
as for security: i always move the files after a few days to a different folder instead of deleting it. no reuploading needed that way if a client needs it back online.
and, it's the clients responsibity to whom they give the downloadlink. i once got a call from a client asking why the downloadlink stopped working after a week. it turned out they published the url in a national press release, if i could please put it back online for a few weeks more