The Z catalog weighs less than 1 lb. and therefore is less expensive to mail.
The H book weighs more than 16 oz., so the postage is based on zone pricing. The farther the destination is from Ripon, Wisconsin, the more the postage will be.
You can mail any quantity, but will receive better per piece postage rates when you mail at least 300 pieces. Many times it will be cheaper to mail 300 pieces than it is to mail 292 pieces. So if your list is close to 300 addresses, we would advise you to add names to your list.