All nouns ending in the "s" sound add the "iz" sound to form the plural. It doesn't matter how the word is actually spelled; it's the sound that counts.
(The same is true for nouns that end in the "z", "ch", "sh", "x", or "j" sound.)
faces, misses, poses, fizzes, witches, riches, wishes, brushes, boxes, grudges
CJ