Airlines don't have "stewardesses" on their payroll and haven't for decades. You can call anybody any job title you'd like, but most people appreciate having their job title be told correctly. Especially when the wrong title can be seen as derogatory.

Also it sounds like they are just using off the shelf software, not trying to extort gratuities. It's possible that it just ignores whatever you put into the tip field anyway, and charges for only the base price.