I just use excel for my invoices, and export it to make a PDF so it can't be altered. It's pretty simple, and does its job.

I hardly track my individual expenses flying because it's a side job and I rarely cover anything myself. For my "real" job of farming, we use quicken to track everything and I have a lot of categories i break expenses into, including personal and things for the family we can get deductions/credits on.

A lot of people like quick books, but we have taken the "if it's not broke, don't fix it" approach because quicken has been enough for us, and it's easy to use.