They are not prewashed. Charm packs are usually 40-42 5" squares, but can be less. They retail here at my LQS for $9.00, so one for $6 with $3 to ship isn't a bargain unless local availability is an issue. Jelly rolls are 42 2 1/2" WOF strips. I think they retail for over $30. About the same price as the fabric. The advantage is the mixed selection and no cutting. Honey Buns are an 1 1/2" strips. I think layer cakes are 10" squares.
You can control the quality by only buying from lines you trust like Moda.
Connectingtreads.com does not charge to cut fabric - their charms, etc are priced like their fabric, but some is only medium quality. Not a bad thing, but one to be aware of.