Prefix and prix fixe are easily confused words.
The spell-check application in word processing programs and the autocorrect function on mobile devices doesn’t necessarily catch a slip-up of these two words. Even if autocorrect did catch a slip-up, it wouldn’t necessarily supply the right word you meant to type.
Prefix is a word describing the 1-4 letter terms that are attached to the beginning of another word to change its meaning. A list of common English prefixes, and their meanings, can be found here. Prefix can also describe items arranged in advance, or placed in front like a title before someone’s name.
Prix Fixe isn’t an English term. At first look, it seems like it would be pronounced “pricks fix”, but it’s French, so the “ix” has an “ee” sound. Prix fixe is a restaurant world term that means “fixed price” or “set price”. It is a menu with only 3-4 options offered per course. The diner picks one selection for each course. Typically there are no substitutions available on prix fixe menus. [TRIVIA: In fine dining environments in the US, you’re bound to run into lots of French and Italian terms on your menu.]
The following story uses both words correctly:
Petunia was perplexed by her recent fine dining experience and her esteemed company. Everyone had prefixed Mr. or Ms. in front of their first name, and instead of an appetizers, main course and dessert chosen separately, they ordered off a prix fixe menu.