I think that the charm packs are only the amount of fabric in that line. So if that line only has 24 fabrics then you get 24, if they have 18 you get 18.
They charge for them at least online they charge per yard for them.
Example:
So how ever many charms equal 1 yard is how they price it if it takes 50 charms to make a yard (pulling a figure out of my head I dont really know) and a yard is 5.00. Then if you got 25 charms (the amount in that line of fabric) then it would be 2.50