Screencasts (at $10-30 per sale) and online training ($200-700 per seat) are the bulk of my such sales and I've had only a few refunds on mid 5 figure sales of both.
In every case it was because they bought it by accident or wanted to change to a different level. I don't believe I've had any people request refunds out of dissatisfaction or with no straightforward reason. But my audience is professional software developers so I'd expect the rates to be pretty low.