As far as I know, there is no way to do that. You could maybe use another field to record the last name and then sort by that column in your invoice or customer list. If you're not using the group or salesperson fields, one of those would work. It's a workaround. I have wished for years that there was a way to do that. This workaround doesn't help when trying to select a name in a drop down field, which is a time you really need that.
You can submit a suggestion to NCH for this feature in the Help menu of your program.
Teri