Spatial relations are not my strong suit. Or even my mediocre suit. I recently purchased dried lavender to use as filler for sachets. I am making about 25 sachets around 4"x4". The website clearly states that a 1 pound bag holds 13.5 cups so I can't explain why I felt I needed to order 3 bags. The bags are probably 7"x7"x3" which is going to be plenty. More than plenty.
If you can find lavender locally, that's great. But if you can't the best deal I found was here.