See Inside Germs (See Inside Books)

A "package" is made up of two or more items sold as a set, often for a reduced price. A listing of individual items that make up a package is provided on the package item's product detail page along with real-time item availability of those items.
Any item sold as a package on our website is identified by a unique alpha-numeric item number (such as "APH1AB"). Although packages are sets, items are not physically bundled together.. Because most package items or components are also sold separately and may be components of multiple packages, these items may not have the same inventory availability at any point in time. When you order a package, you are charged one price for all package items. In-stock components will ship according to our normal shipping time. Any backordered components will ship separately as they become available. On your packing slip, package components are picked and packed individually and are identified with the code "PKGCMP" in the price column. The package item number is also listed at the bottom of your packing slip for reference. Please note that if your order ships in multiple boxes, package components may not all be in the same box. Package content is not flexible and cannot be modified.