Hello,

Cassis -> don't pronounce the final "s"
Pastis -> do pronounce it
Mas -> do pronounce it

or at least that's what most of the Provençaux would tell you (people who live in Cassis are called "Cassidains" although you'll seldom read this word anywhere). As always, there's always exceptions, but mostly these exceptions are people from the North part of France who don't have a clue

Cassis is, litterally, the word for blackcurrant, but that's just a coincidence. The name of the city derives from latin "Carsicis" then "Castrum Cassitis". The word 'cassis' in French is also for a speed bump on the road (also called "dos d'âne", donkey back - beware if you see this on a sign) or a type of sea gasteropod. Which is found in Bouillabaisse. (OK, I'm kidding on that one)

Cheers,
J-