Customers find it annoying that they have to re-populate the remote fax station id every time.
Perhaps there could be a send fax preferences where you can set defaults.
Also in this area could potentially be a cover sheet that they can upload once and have attached to all of their faxes.
The customer is finding it cumbersome to concatenate their fax and their coversheet into a single pdf before every fax.
Some type of remembering numbers that have been faxed to so you do not have to type in the full number every time, which is also error prone.