The differences in U.S. Postal Service shipping prices are due to the size of the package, the type of service the person is choosing and the distance to the mail's destination. The weight of the package also plays a role in the difference in postal prices.
The USPS attaches different postage prices to large flat envelopes, parcels and postcards. Generally speaking, the larger the package, the more it costs to mail it. First-class mail and postcards cost less than letters to mail. Flat-size mail is more costly than letter-size mail. A parcel costs the most to mail due to the price of processing and delivering the mail. There is no postcard price in standard mail. Postcards are mailed as standard mail, but the cost is the same as a letter.
There are different classes of mail that determine the differences in postal rates. The classes also determine what can be mailed and what the minimum and maximum weights are. Priority Mail Express is available for letters as well as merchandise. Priority Mail and first-class mail are available for bills, personal correspondence, merchandise and invoices. Periodicals rates are for magazines and newsletters, and standard mail includes circulars, newsletters, small parcels and merchandise.