Putting aside the photo print, a tangible object, isn't the very nature of digital photography a market based on licensing not selling? The photographer keeps the copyright, unless you truly sign away your copyright, even with a simple headshot portrait for a $1 that is a licensing transaction. It's the nature of photography. If you give someone a digital picture you are not giving them a tangible object you are allowing them the unlimited licensing rights to the reproduction of that image.
If a client wants the licensing rights to archive your photos then they need to license the images for that agreement, granted that could be at a lower fee than the rights to publish on Facebook etc.. but it has nothing to do with whether you are going to do any additional post work to the images. And your contract should spell out that there is no social media publishing or other rights such as printing rights included.
I don't know your contract but I do believe all photographers should be charging some type of online delivery fee, drop box is not free nor is web hosting nor the amount of time to upload files or your own internet charges to do so.