They do it because it is more convenient for them.  But it isn't so convenient for us.  In all seriousness, for those clients that are local, consider charging a convenience fee for those that use the portal.  We accept credit cards, but the majority of those that use plastic is because they get points or miles.  Well, my fees are very reasonable (too low) so I'm not going to eat the fees because you want to earn some extra benefits by paying that way.  I charge a convenience fee for the use of plastic.  It's a nominal fee but at least it recoups most of the fees that I would get clipped for by the end of tax season.