question: Why do package contents makes mail rates so different, even if weight is similar?
comment: there's a special rate for books called media mail. its cheaper than the regular rate. but if you are using the same class of service, ie usps priority mail, express mail, flat rate, package, etc whatever. then similar size and similar weight will be similar price. but likely the books were media mail and the other one was priority mail.
passages: BULLET::::- DHL Express
  
  BULLET::::- FedEx
  
  BULLET::::- United Parcel Service
  
  BULLET::::- United States Postal Service
  
  Section::::Shipping companies.:Examples.
  
  Using dimensional weight calculations, a freight carrier will charge for lightweight (low density) packages as if they had a greater weight (the weight of the package at the minimum accepted density). 
   BULLET::::- Third and Fourth Class Mail became Standard Mail (A) and (B)
  
  BULLET::::- Special Fourth Class Mail was renamed Special Standard Mail
  
  In 2007, First Class Mail was restructured to include variable pricing based on size, not just on weight. Shape-based postage pricing is a form of dimensional weight. Also at that time, International Parcel Post air service was re-branded as Priority Mail International, and Parcel Post surface service was discontinued for international destinations.
   First-Class Mail in the U.S. includes postcards, letters, large envelopes (flats), and small packages, providing each piece weighs or less. Delivery is given priority over second-class (newspapers and magazines), third class (bulk advertisements), and fourth-class mail (books and media packages). First-Class Mail prices are based on both the shape and weight of the item being mailed. Pieces over 13 ounces can be sent as Priority Mail. As of 2011 42% of First-Class Mail arrived the next day, 27% in two days, and 31% in three. The USPS expected that changes to the service in 2012 would cause about 51% to arrive in two days and most of the rest in three.
   Letter-sized mail constitutes the bulk of the contents sent through most postal services. These are usually documents printed on A4 (210×297 mm), Letter-sized (8.5×11 inches), or smaller paper and placed in envelopes.
   In May 2007, the United States Postal Service (USPS) adopted dimensional weight, calling it "Shape Based Postage Pricing". This rate system is designed to charge more for lightweight items, and also to recover costs involving manual sorting and handling, since many postal machines are built to handle flats. This system would charge much more for mailing a parcel than a flat envelope. It encourages mailing books and DVDs in flat paperboard or plastic envelopes, rather than padded mailers.
   Shipping costs have historically been calculated on the basis of gross weight in kilograms or pounds. By charging only by weight, lightweight, low density packages become unprofitable for freight carriers due to the amount of space they take up in the truck/aircraft/ship in proportion to their actual weight. The concept of dimensional weight has been adopted by the transportation industry worldwide as a uniform means of establishing a minimum charge for the cubic space a package occupies. In fact, UPS and FedEx both announced that starting 2015, shipping charges on all shipments (air and ground) will be determined by greater of the actual weight and dimensional weight of a package. Prior to this announcement, dimensional weight charges were only applicable to packages of a specific size range.
   Larger envelopes are also sent through the mail. These are often composed of a stronger material than standard envelopes and are often used by businesses to transport documents that may not be folded or damaged, such as legal documents and contracts. Due to their size, larger envelopes are sometimes charged additional postage.
   Shipping factors for imperial measurements represent cubic inches per pound (in/lb) while metric factors represent cubic centimeters per kilogram (cm/kg). These are the inverse of the package density. Dimensional weight is applied when the actual product density is less than the minimum density represented by the chosen factor. Dimensional weight is representative of the weight of the package at the minimum density accepted by the freight carrier. Shipping factors are not only different for imperial and metric measurements, but also for shipment mode and in some cases between different customers. Shipping factors will be available from the freight carrier. Some common factors are listed below. 
   One of the drawbacks of mail is the uneven weight distribution; the stress falls mainly on shoulders. Weight can be better distributed by wearing a belt over the mail, which provides another point of support.
   BULLET::::- The traditional "minimum weight" system where weights below a specified weight are rejected. Normally the minimum weight is the weight that is printed on the pack or a weight level that exceeds that to allow for weight losses after production such as evaporation of commodities that have a moisture content. The larger wholesale companies have mandated that any product shipped to them have accurate weight checks such that a customer can be confident that they are getting the amount of product for which they paid. These wholesalers charge large fees for inaccurately filled packages.
   Mail sack
  
  A mail sack or mailsack is a mail bag used to carry large quantities of mail.
  
  Different handling and security requirements for different classes of mail is integral to the postal rate structure.
   BULLET::::- Books and recorded media only.
  
  BULLET::::- No advertising.
  
  BULLET::::- Pricing by weight only.
  
  BULLET::::- Transit time similar to Parcel Post.
  
  BULLET::::- Cheaper than Parcel Post but only due to increased restrictions on package contents.
  
  BULLET::::- Library Mail
  
  BULLET::::- Similar to Media Mail, but cheaper and restricted to academic institutions, public libraries, museums, etc.
  
  The Post Office will not deliver packages heavier than or if the length (the package's longest dimension) plus the girth (the measurement around the package at its largest point in the two shorter dimensions) is greater than combined ( for Parcel Post)
   It has been observed that the general trend in mail deliveries is a decrease in letter volume and an increase in the number and size of packages; bulk and weight increased so mail delivery methods had to be changed as a result. In one intracampus mail system, it was found that the use of Segways decreased emissions. "Increasingly heavier trays were leading to workplace accidents, exhaustion and dissatisfaction." Segway PTs fitted with custom mail bags, a derivative of the over-the-shoulder personal mail satchels, were seen as a cure for the postmen for many of these problems.
   However, there are a number of factors that limit the parcel size :
  
  BULLET::::- The stock level;
  
  BULLET::::- Available depth;
  
  BULLET::::- Decreasing economies of scale in larger ships.
   Packages are often sent through some postal services, usually requiring additional postage than an average letter or postcard. Many postal services have limitations as to what a package may or may not contain, usually placing limits or bans on perishable, hazardous or flammable materials. Some hazardous materials in limited quantities may be shipped with appropriate markings and packaging, like an ORM-D label. Additionally, as a result of terrorism concerns, the U.S. Postal Service subjects their packages to numerous security tests, often scanning or x-raying packages for materials that might be found in biological materials or mail bombs.
   Shipping costs have historically been calculated based on weight alone. By charging only by weight, lightweight, low-density packages become unprofitable for freight carriers due to the amount of space they take up in trucks, aircraft, and ships in proportion to their actual weight. By using dimensioning technology to calculate an item's dimensional weight, carriers are able to charge based on either volume or weight, depending on which is greater.
   Dimensional weight
  
  Dimensional weight, also known as volumetric weight, is a pricing technique for commercial freight transport (including courier and postal services), which uses an estimated weight that is calculated from the length, width and height of a package.
  
  Section::::Significance.
   An example of how package design is affected by other factors is its relationship to logistics. When the distribution system includes individual shipments by a small parcel carrier, the sorting, handling, and mixed stacking make severe demands on the strength and protective ability of the transport package. If the logistics system consists of uniform palletized unit loads, the structural design of the package can be designed to meet those specific needs, such as vertical stacking for a longer time frame. A package designed for one mode of shipment may not be suited to another.
   Section::::United States Postal Service.:Third-class mail.
  
  Third-class mail that would be carried in a "mail sack" is printed material weighing less than 16 ounces. Examples are circulars that are not of a personal nature (e.g. mass general public advertising, direct advertising mailing campaigns). Other third-class mail that would be carried in a "mail sack" is bulk mail that is presorted individually addressed letters that come in quantities of at least 50 pounds or mailings of over 200 pieces.
  
  Section::::United States Postal Service.:Fourth-class mail.
   Large pallet loads, bulk boxes, wooden boxes, and crates can be evaluated by many of the other test procedures previously listed. In addition, some special test methods are available for these larger loads.
  
  Section::::Test Protocols for Shipping Containers.
   The ability of a package to withstand these vibrations and to protect the contents can be measured by several laboratory test procedures. Some allow searching for the particular frequencies of vibration that have potential for damage. Modal testing methodologies are sometimes employed. Others use specified bands of random vibration to better represent complex vibrations measured in field studies of distribution environments.
  
  Section::::Compression.
   Vibration is encountered during shipping (vehicle vibration, rough roads, etc.) and movement on conveyors. Potential vibration damage may include:
  
  BULLET::::- fractures and fatigue damage
  
  BULLET::::- loose wires, screw caps, etc.
  
  BULLET::::- bruises on soft products (fruit, etc.)
  
  BULLET::::- surface abrasion
  
  BULLET::::- etc.
   According to the National Postal Museum, "V-mail ensured that thousands of tons of shipping space could be reserved for war materials. The 37 mail bags required to carry 150,000 one-page letters could be replaced by a single mail sack. The weight of that same amount of mail was reduced dramatically from 2,575 pounds to a mere 45." This saved considerable weight and bulk in a time in which both were hard to manage in a combat zone.
   Each product has particular parcel size in which may be transported in the most economical way. This is expressed as "parcel size distribution". Thus, the "parcel size distribution" size for coal it's completely different from the size used for ore, for grain or for sugar.
   The environmental conditions of testing are critical. The measured performance of many packages is affected by the conditioning and testing atmospheres. For example, paper based products are strongly affected by their moisture content: Relative humidity needs to be controlled. Plastic products are often strongly affected by temperature.
  
presuppositions: Packages with different contents have different mail rates.
corrections:  Most contents have the same mail rate, only books are different.
raw_presuppositions: Packages with different contents have different mail rates. Packages with different contents have different mail rates.
raw_corrections:  Most contents have the same mail rate, only books are different.  Most contents have the same mail rate, only books are different.