These words are pronounced the same way, but are otherwise unrelated and have different meanings.
Sachet is a French noun meaning little sack (a sack is a bag). Sachet most often refers to very small bags of herbs or perfumes that you can use to make a nice smell in places where clothing is stored. It can also describe any bag smaller than a backpack.
Sashay is mainly a verb, coming from a French word for a ballet step (chasser). In English, it describes a manner of walking which is both graceful, and showy—fluid and bold. As such, it can have two meanings: (1) a manner of walking, sliding and moving from side to side, like an ice-skater, or (2) an attitude of casual boldness and confidence when walking:
She sashayed into the room as if she had no cares in the world. (attitude)
He sashayed around the obstacles gracefully. (motion)
Sashay can also be used, as a noun, for a short trip—an excursion:
I’m going to make a quick sashay to the store for potato chips.